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PerspectiveNovember 29, 2024

Sum-Frequency Generation Spectroscopy of Aqueous Interfaces: The Role of Depth and Its Impact on Spectral Interpretation
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Alexander P. Fellows*
Alexander P. Fellows
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
*Email: [email protected]. Tel: +49 (0)30 8413 5140.
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https://orcid.org/0000-0002-5885-8144
Álvaro Díaz Duque
Álvaro Díaz Duque
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
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Vasileios Balos
Vasileios Balos
Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA Nanociencia), 28049 Madrid, Spain
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https://orcid.org/0000-0001-7606-6653
Louis Lehmann
Louis Lehmann
Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany
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Roland R. Netz
Roland R. Netz
Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany
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Martin Wolf
Martin Wolf
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
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https://orcid.org/0000-0001-7226-9592
Martin Thämer*
Martin Thämer
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
*Email: [email protected]. Tel: +49 (0)30 8413 5220.
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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2024, XXXX, XXX, XXX-XXX

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https://doi.org/10.1021/acs.jpcc.4c06650

Published November 29, 2024

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Abstract

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Vibrational sum-frequency generation (SFG) has become a dominant technique in the study of molecular interfaces owing to its capabilities for molecular recognition and specificity to anisotropic structure. Nevertheless, one crucial and influential aspect of the interfacial structure, namely, its inherent three-dimensional, depth-dependent nature, cannot be obtained through conventional SFG measurements. Furthermore, not only has this depth information been so far experimentally inaccessible through SFG, the simple existence of extended anisotropic depth also complicates the analysis and interpretation of any obtained spectra. In this Perspective, we analyze the role of depth-dependent structural anisotropy in second-order vibrational spectroscopy and explore various possibilities for how the desired depth information can be experimentally attained. Using aqueous interfaces as an important and widespread example system, we highlight the prevalence of such spatially extended depth profiles, demonstrate how signals from these regions can cause significant spectral distortions, and show the entanglement between experimental parameters with the overall nonlinear response. Finally, we evaluate recently developed measurement concepts that can yield depth information, emphasizing their particular strengths, and provide an outlook for future studies employing these methodologies for the vital elucidation of depth-dependent interfacial structure.

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Special Issue

Published as part of The Journal of Physical Chemistry C special issue “Alec Wodtke Festschrift”.

1. Introduction

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Molecular interfaces are widespread across many natural and technological processes, where the unique properties of the interface strongly control their underlying function. (1−5) A central factor in determining these unique properties is the structure and dynamics of the interfacial molecules which, in contrast to the bulk, exhibit altered kinetics, orientational distributions, and intermolecular environments. Isolating the properties of this thin molecular boundary is, however, a veritable challenge requiring specialized techniques. One such technique that has been particularly elucidative is vibrational sum-frequency generation (SFG) spectroscopy. As a second-order optical process, the sign of the output SFG response (under the electric dipole approximation, EDA) is dependent on the molecular orientation, making any signals from structurally isotropic environments vanish. (6−8) For most media, this renders the technique interface-specific (in the dipolar limit), making it an excellent probe of the anisotropic interfacial structure. In addition to these symmetry selection rules, the output SFG signals also yield characteristic vibrational spectra for the molecules being probed, giving the technique molecular recognition and environment sensitivity.

While SFG investigations have led to many crucial revelations about the structure of a wide range of interfaces, (9−19) the molecular level interpretation of their spectra is nevertheless highly demanding, necessitating an exceptional understanding of the underlying theoretical concept giving rise to the signals. In this context there are two specific challenges, the first being to obtain a characteristic quantity (i.e., a material parameter) that is independent of the experimental settings and contains the desired structural information on the interface, and the second is then actually extracting the structural information from this parameter. For SFG, the material parameter governing the output signals is the second-order susceptibility, χ⁽²⁾. (6−8,20) Therefore, the ultimate aim of SFG measurements is to extract χ⁽²⁾ for the interface, more precisely its frequency dependent dipolar, vibrationally resonant part. This task is already a formidable challenge due to the common presence of nonresonant and quadrupolar signal contributions. (7,21,22) However, an additional great challenge is to remove the influence of specific experimental settings on the measured spectra, and to correctly interpret the resulting spectral information in terms of the interfacial structure. For completely homogeneous non-centrosymmetric media, this may be possible, but as soon as there is any spatial inhomogeneity, especially as a function of the distance from the interface (depth, z), χ⁽²⁾ will depend on the z-coordinate. In fact, in basically all applications where SFG spectroscopy is performed to investigate interfacial properties, χ⁽²⁾ decays to zero toward the bulk on some characteristic length-scale. As a result, the desired spectroscopic quantity that is needed to properly describe the interfacial properties is at least two-dimensional (χ⁽²⁾(ω, z)). In a regular SFG experiment, however, it is not this two-dimensional quantity that is measured, but its (along z) integrated form, the effective susceptibility of the interface. Any information on the evolution of the nonlinear signal with depth is consequently lost, and with this, a large portion of the desired structural insight. Furthermore, this integration can also lead to important distortions of the vibrational line-shapes which obstruct an unambiguous interpretation of the obtained spectra. Finally, the integration in z leads to an intrinsic convolution between the nonlinear responses and experimental parameters (such as angle(s) of incidence) that cannot be disentangled without precise depth information. In order to obtain a fully unbiased view on interfacial molecular structures it is therefore clear that suitable experiments should additionally include depth information.

In this Perspective, we directly discuss the impact of the depth dependency of χ⁽²⁾(ω, z) in SFG investigations, with a specific focus on one particularly prevalent class of interfaces, namely those involving water. It is divided into four main sections. In section 2, the role of depth is examined by considering its connection to the accessible quantity in SFG measurements, with examples discussed for the pure air–water interface as well as those with a net surface charge. In doing so, we also explicitly demonstrate the entangled nature of the material properties with experimental parameters, showing how signal contributions from nonzero depth generally lead to their inseparability. Thereafter, in section 3, we critically explore how structural information can be extracted from measured SFG spectra, focusing particularly on both the nonresonant contribution and quadrupolar mechanisms, showing how they are linked to depth and can alter the structural interpretations of any obtained spectra. With these important discussion points in hand, we review in section 4 the latest developments in SFG spectroscopy in terms of overcoming some of these complications and gaining access to depth information. Finally, in section 5, we then discuss the potential of these methods in the perspective of the future directions for more advanced structural interpretations at aqueous interfaces.

2. The Importance of Depth in SFG Measurements

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The structural anisotropy at interfaces and its evolution with depth is a prominent and defining parameter controlling many properties. Due to its selection rules, SFG primarily probes this anisotropy, so the depth-dependent χ⁽²⁾(z) that governs the output SFG signals is directly representative of the depth-dependent anisotropy at the interface. In this section, we present the structural insight that can be gained by accessing χ⁽²⁾(z), and analyze the relation between the measured signal and this quantity, as well as how depth is intrinsically intertwined in their relation.

2.1. The 2D “Picture” of the Interface

State-of-the-art molecular dynamics (MD) simulations can generally yield the complete spatially resolved (2D) susceptibility (χ⁽²⁾(z)), and thus give a “full” picture of the interfacial region. (23−25) An example of this is shown in Figure 1 for the air–water interface, including a snapshot of an ab initio-parametrized MD simulation of the structural transition between the two phases and the calculated depth and frequency-resolved 2D second-order susceptibility, shown in the O–H stretching region.

Figure 1. Snapshot of the air–water interface and the corresponding depth-dependent density and second-order susceptibility calculated from molecular dynamics simulations. The depth-dependent susceptibility highlights the regions with anisotropic structure within the interfacial layer. GDS: Gibbs dividing surface.

This 2D picture of the interfacial structure clearly shows the same spectral features as observed from previous phase-resolved SFG measurements of the air–water interface, (24,26−30) with a sharp positive (yellow/red) resonance at ∼3700 cm^–1 arising from non-H-bonded (“free”) OH groups pointing “up” toward the air phase and a broader negative (cyan/blue) resonance at ∼3400–3600 cm^–1 from H-bonded OH groups pointing “down” toward the bulk water phase. Beyond this, however, far more insight can be obtained from this spatially resolved picture. For example, the entire SFG signal arises from a region of ∼6 Å, corresponding to 2–3 molecular layers, showing that the entire structural anisotropy is restricted to this region. Furthermore, it is clear that both of the aforementioned moieties exist through this entire region since both the positive and negative signals appear from the same depths. This shows, for example, that the picture of a sharp transition between the gas and condensed phases with the topmost condensed layer containing all of the “free” OH and the molecules beneath solely contributing strongly H-bonded signals is clearly inaccurate. (30−33) On closer inspection, there is some slight indication of the “free” OH signal starting from closer to the air-phase than the H-bonded signal, aligning with expectation, but also that both contributions appear to somewhat red-shift on increasing depth, indicating a gradient in H-bonding strength when approaching the bulk. This observation is in line with the idea of the interface having a “healing depth” over which the loss of H-bonding induced by the interface is recovered. (24,31) Overall, Figure 1 clearly showcases the complementary information obtained by comparing the simulation snapshot and its associated temporally averaged density profile with the calculated second-order susceptibility χ⁽²⁾(z). The former clearly embodies the molecular density, its inhomogeneities, and specific local structural motifs. However, the anisotropy in molecular orientation and interconnectivity only becomes apparent upon inspection of the χ⁽²⁾(z) image.

2.2. The Experimentally Accessible Quantity in SFG

Evidently, having access to the depth dependency of χ⁽²⁾(z) would provide a highly detailed structural view of the interface. Regular SFG measurements, however, do not yield this two-dimensional quantity, but rather a single spectrum of the output SFG field that often has some characteristic line-shape from which structural information can be inferred. As it is not the second-order susceptibility that is measured, it is thus important to know how the obtained SFG signals precisely relate to the intrinsic properties of the medium, i.e., what exactly is the measured quantity in an SFG experiment?

Generally, the output SFG field (under the EDA) is produced by the induced second-order polarization oscillating at the SFG frequency, p⁽²⁾(ω₃ = ω₂ + ω₁), which is generated by the coupling of the two incident fields, E₁ and E₂, as in eq 1,

p^{(2)} (ω_{3} = ω_{2} + ω_{1}) = ϵ_{0} χ^{(2)} (ω_{3}, ω_{2}, ω_{1}) E_{2} E_{1}

(1)

where ϵ₀ is the vacuum permittivity. (6−8,20,34,35) Importantly, however, the detectable quantity rather arises from the sum of all individual second-order polarization in the medium, hence represented by a 3D spatial integral, as in eq 2.

P_{eff}^{(2)} = \int_{V} p^{(2)} d V

(2)

Given that the vast majority of substrates, including aqueous systems, possess in-plane homogeneity, contributions from the same depth (distance from the interface) are only modulated by the lateral field distribution (e.g., spatial beam profile and spot size). Thus, the lateral integration in eq 2 can be typically reduced to a beam-dependent amplitude prefactor. For contributions arising from different depths, however, not only are the local structures potentially different, leading to differing induced local second-order polarizations, but the input and output beams also have an increased path length. This increased beam propagation results in a depth-dependent phase shift (propagation phase) that is linked to the z-component of the wave-vector mismatch (reciprocal of the coherence length), Δk_z, which must be incorporated into the spatial integral. (24,36−38) Overall, therefore, the detectable output field is actually governed by an effective second-order susceptibility, χ_eff⁽²⁾, that is given by eq 3.

\begin{matrix} χ_{eff}^{(2)} = \int_{0}^{\infty} f_{3} (z) f_{2} (z) f_{1} (z) χ^{(2)} (z) e^{i Δ k_{z} z} d z \end{matrix}

(3)

This definition also includes the local field correction factors, f(z), to account for the dielectric inhomogeneity which must also be present at the boundary between the two bulk phases. (7) As the integral in eq 3 cannot be generally solved, the accessible quantity in SFG measurements is restricted to this integrated quantity. In consequence, the entire depth information is inaccessible through regular SFG measurements.

2.3. Obtaining Absolute Units

While the effective susceptibility given in eq 3 represents the accessible quantity in SFG measurements, it is still not the measured quantity, as this also includes the strength of the incident fields and an instrument-dependent prefactor. In order to extract the pure χ_eff⁽²⁾ from a measurement, therefore, the measured signals must be referenced. This is usually achieved by comparing the measured signal to the analogous output from a bulk nonlinear crystal. (39) In doing so, therefore, the obtained spectra can also be converted into absolute units by using the known value for the second-order susceptibility of the reference, e.g., for z-cut quartz, the aaa tensor component of the second-order susceptibility (using the coordinates defined by its crystal symmetry) is widely accepted to be 6 × 10^–13 m V^–1. (8) As the effective susceptibilities for both sample and reference are depth-integrated quantities, and therefore formally not true second-order susceptibilities, it is important to notice that they also possess different units: m² V^–1 instead of m V^–1 as for the regular second-order susceptibility. Therefore, for proper referencing and obtaining absolute units for the sample response, the known reference susceptibility must also be combined with experimental parameters (e.g., angle(s) of incidence) and integrated, just as in eq 3, effectively leading to a coherence length-modulated amplitude.

The two responses, sample (S^s) and reference (S^ref), can be written as in eqs 4 and 5, respectively, with the indices i, j, k representing the specific spatial components of the fields and susceptibility tensor, and L_n being the Fresnel factors for the input and output beams which account for the change in field amplitudes between the incident and sample medium.

S^{s} \propto I_{s} (θ) L_{i}^{s} (ω_{3}) L_{j}^{s} (ω_{2}) L_{k}^{s} (ω_{1}) \times \int_{0}^{\infty} f_{i}^{s} (ω_{3}, z) f_{j}^{s} (ω_{2}, z) f_{k}^{s} (ω_{1}, z) χ_{i j k}^{(2), s} (z) e^{i Δ k_{z} z} d z E (ω_{2}) E (ω_{1})

(4)

S^{ref} \propto I_{ref} (θ) L_{i}^{ref} (ω_{3}) L_{j}^{ref} (ω_{2}) L_{k}^{ref} (ω_{1}) \times \int_{0}^{\infty} f_{i}^{ref} (ω_{3}, z) f_{j}^{ref} (ω_{2}, z) f_{k}^{ref} (ω_{1}, z) χ_{i j k}^{(2), ref} (z) e^{i Δ k_{z} z} d z E (ω_{2}) E (ω_{1})

(5)

Here, these signals are given neglecting the instrument-dependent proportionality constant (being the same for both sample and reference), but include a prefactor, I(θ), to account for the dependency of the output on the incident angle, with its functional form also depending on the specific tensor element being probed (i.e., the spatial components of the incident and output fields). As an example, using quartz as a reference in the commonly used SSP polarization combination, eqs 4 and 5 can be rewritten as in eqs 6 and 7. Note, as the susceptibility for the reference is constant, the integral can be directly solved.

S^{s} \propto \sin θ L_{y}^{s} (ω_{3}) L_{y}^{s} (ω_{2}) L_{z}^{s} (ω_{1}) \times \int_{0}^{\infty} f_{y}^{s} (ω_{3}, z) f_{y}^{s} (ω_{2}, z) f_{z}^{s} (ω_{1}, z) χ_{y y z}^{(2), s} (z) e^{i Δ k_{z} z} d z E (ω_{2}) E (ω_{1})

(6)

S^{Q} \propto \cos θ L_{y}^{Q} (ω_{3}) L_{y}^{Q} (ω_{2}) L_{x}^{Q} (ω_{1}) \times \int_{0}^{\infty} f_{y}^{Q} (ω_{3}, z) f_{y}^{Q} (ω_{2}, z) f_{x}^{Q} (ω_{1}, z) χ_{y y x}^{(2), Q} (z) e^{i Δ k_{z} z} d z E (ω_{2}) E (ω_{1}) = L_{y}^{Q} (ω_{3}) L_{y}^{Q} (ω_{2}) L_{x}^{Q} (ω_{1}) \frac{i}{Δ k_{z}} χ_{y y x}^{(2), Q} E (ω_{2}) E (ω_{1})

(7)

Here, the SSP polarization combination probes both the YYX and YYZ tensor components, with the former vanishing in the case of in-plane isotropy. (6−8,20) Therefore, for typical sample interfaces such as aqueous systems, the YYZ component is the only contribution. In contrast, z-cut quartz yields the YYX contribution in SSP. (40) Division of the two responses then yields the expression in eq 8 that has removed any field dependency and thus provided a known amplitude and phase reference.

\frac{S^{s}}{S^{Q}} = \tan θ \frac{L_{y}^{s} (ω_{3}) L_{y}^{s} (ω_{2}) L_{z}^{s} (ω_{1})}{L_{y}^{Q} (ω_{3}) L_{y}^{Q} (ω_{2}) L_{x}^{Q} (ω_{1})} \frac{Δ k_{z}}{i χ_{y y x}^{(2), Q}} \int_{0}^{\infty} f_{y}^{s} (ω_{3}, z) f_{y}^{s} (ω_{2}, z) f_{z}^{s} (ω_{1}, z) χ_{y y z}^{(2), s} (z) e^{i Δ k_{z} z} d z

(8)

Rearrangement of eq 8 then yields the accessible quantity of interest for the sample (χ_eff⁽²⁾), as in eq 9.

χ_{eff}^{(2)} = \int_{0}^{\infty} f_{y}^{s} (ω_{3}, z) f_{y}^{s} (ω_{2}, z) f_{z}^{s} (ω_{1}, z) χ_{y y z}^{(2), s} (z) e^{i Δ k_{z} z} d z = \cot θ \frac{L_{y}^{Q} (ω_{3}) L_{y}^{Q} (ω_{2}) L_{x}^{Q} (ω_{1})}{L_{y}^{s} (ω_{3}) L_{y}^{s} (ω_{2}) L_{z}^{s} (ω_{1})} \frac{i χ_{y y x}^{(2), Q}}{Δ k_{z}} \frac{S^{s}}{S^{Q}}

(9)

This expression can then be used to yield absolute units by using known values for the susceptibility of z-cut quartz (taken to be 6 × 10^–13m V^–1, as mentioned above), the wavevectors of the incident beams, and calculated Fresnel factors (e.g., using the two- or three-layer model (7)). Importantly, eq 9 highlights that referencing by simple dividing the sample spectrum by the quartz response does not correctly scale the amplitudes. This is clear, for example, as the amplitude scaling of the quartz response is dependent on the coherence length and thus is highly sensitive to the experimental settings and the specific probing frequencies. Therefore, by simply dividing by the quartz response, this dependency on input frequencies and experimental settings enters into the obtained sample spectrum. Instead, by implementing the referencing shown in eq 9, these effects are accounted for, along with the impact of the incidence angle and Fresnel factors.

The above discussion shows that proper referencing is not a straightforward task, requiring not just a simple division but also the incorporation of the beam geometry and frequency-dependent coherence length and Fresnel factors. Nevertheless, reporting spectra in absolute units (m²V^–1) has crucial benefits as it enables better comparisons between the results from different experimental setups, as well as testing the accuracy of simulations. In general, the absolute amplitude of the susceptibility is determined by three factors. First, there is obviously the intrinsic size of the molecular hyperpolarizability, second, there is the density of noncanceling chromophores per molecular layer, which reports on the amount of orientational order, and finally there is the number of contributing molecular layers, i.e. the anisotropic depth. Therefore, being able to compare the absolute amplitudes of SFG signals from different systems gives some insight into these structural parameters. In principle, different samples can be categorized as one of three distinct cases: (i) those with only a single (or very thin) contributing layer, (ii) those with effectively infinite contributions in depth (e.g., for non-centrosymmetric crystals such as quartz), and (iii) anything in between.

An example of comparing two systems in the category (i) is given in Figure 2 which shows the imaginary parts of the SFG spectra recorded in the O–D stretching region for the air-D₂O interface and the C–H stretching region for a self-assembled monolayer of octadecyltrichlorosilane (OTS) formed on fused silica. These spectra are first shown alongside one another in arbitrary units (black traces) in a similar fashion to what is commonly presented in the literature. (41−47) Only when compared in absolute units however (as shown underneath in red), does it become clear that the OTS CH₃ resonances from the terminal methyl group that dominate the spectrum are substantially stronger than the signals from the air–water interface. This is remarkable given that the O–D stretch response of water has a significantly larger molecular hyperpolarizability than aliphatic C–H stretches, similar interfacial density, and even has 2–3 contributing layers rather than just 1, as for OTS. (24) The weak signals from water thus show that the interfacial structure must contain either a significant amount of canceling out-of-plane contributions, suggesting a broad orientational distribution, or a substantial in-plane structure (which also cancel and yield no net SFG signal). It is therefore clear that there cannot be a significant preferential out-of-plane orientation of water molecules at the interface. These observations align with expectation and other findings from simulations, which suggest that a majority of the water dipoles at the interface lie in-plane, and the residual out-of-plane populations are almost equally distributed between “pointing up” and “pointing down”. (33) However, such insight into the structural properties of the interface only becomes possible from an analysis of the SFG signals reported in absolute values.

Figure 2. Imaginary parts of the heterodyned SFG spectra recorded in the SSP polarization combination for the air-H₂O interface in the H–O–H bending region, air–D₂O interface in the O–D stretching region, and OTS self-assembled monolayer on fused silica in the C–H stretching region. Reprinted in part with permission from ref (24). Copyright 2024 American Chemical Society.

While this analysis of the SFG amplitudes from systems in category (i) can be used to obtain important structural information, this can, however, not be generalized to include other systems in category (iii). In such cases, the amplitudes are modulated both by the orientational distribution and the thickness of the anisotropic structure (number of contributing layers). Therefore, without depth information, insight into the orientational distributions cannot be extracted.

2.4. Charged Interfaces

Prominent examples with χ⁽²⁾(z) extending up to 100s of nanometers in depth (i.e., category (iii) above) are charged aqueous interfaces, where the electric field induces far-reaching structural anisotropy. These are found in numerous relevant systems ranging from physiological membranes and oceanic surfaces, to electrochemical devices, thus being of enormous scientific and technological interest. (48) As such, many SFG investigations have been performed on charged interfaces to elucidate their structural and dynamical properties. (14,48−53) As the role of depth in the SFG spectra from such systems becomes immensely influential, however, interpretation of the obtained data remains challenging.

The presence of such excess surface charge generates a static electric field which can penetrate into the bulk water and act on any electric charges or dipoles, resulting in altered ion distributions and, importantly, the alignment of strongly dipolar water molecules. This field-induced orientational order within the otherwise isotropic water environment can thus give rise to SFG signals originating several 100s of nm away from the interface. (38)

The theoretical description of the response from charged interfaces can be found in many places in the literature. (38,51,54−56) However, it is important to note that in many cases, the presented derivations are shown with a final equation that neglects aspects about the depth-dependent electrostatics in such systems and their connection to the SFG responses. (38,51,55,56) It is therefore important to present the derivation and its associated assumptions in full detail so as to avoid any future misconceptions.

If we include the field-induced response, the effective susceptibility becomes as in eq 10, with E_DC(z) being the static (DC) field from the interface which is combined with a third-order susceptibility, χ⁽³⁾, to generate SFG photons. (38,49,55) This description comes from including the first term in the Taylor expansion with the DC field, thus in the limit of the response of water to the field being linear.

χ_{eff}^{(2)} = L_{3} L_{2} L_{1} \int_{0}^{\infty} f_{3} (z) f_{2} (z) f_{1} (z) (χ^{(2)} (z) + χ^{(3)} (z) \cdot E_{DC} (z)) e^{i Δ k_{z} z} d z

(10)

The resulting integral can clearly be split into two components, with the first arising from the direct solvation (chemistry driven) χ⁽²⁾ contribution that behaves the same as in the earlier discussion and generally will only be relevant over a few molecular layers, and the second being the field-dependent χ⁽³⁾ contribution which follows the depth of the static electric field, thus potentially representing 1000s of molecular layers. It is this latter contribution, therefore, that makes depth a crucial parameter in SFG measurements from such systems.

In order to evaluate the field-induced contribution, it is important to note that the static field is nothing other than the negative gradient of the electric potential, ϕ, allowing eq 10 to be rewritten as in eq 11.

χ_{eff}^{(2)} = L_{3} L_{2} L_{1} \int_{0}^{\infty} f_{3} (z) f_{2} (z) f_{1} (z) (χ^{(2)} (z) - χ^{(3)} (z) \cdot \frac{d ϕ (z)}{d z}) e^{i Δ k_{z} z} d z

(11)

Clearly, however, to perform the integration, not only does the depth-dependency of both susceptibilities need to be known, but also that of the electric potential. For this, it is common to resort to electrostatic models of the interface such as the Gouy-Chapman-Stern (GCS) model, a schematic of which is presented in Figure 3a. (48,57−62) This shows two distinct electrostatic regimes, the compact layer (CL, often also referred to as the Stern layer, SL, or bonded interfacial layer, BIL (54,56,63,64)) in the immediate vicinity of the surface charges which accounts for direct solvation at the interface, and a second regime beneath, the diffuse layer (DL), which describes the screening of the residual electric field due to dissolved ions in an otherwise bulk-like environment, with the characteristic Debye length, z_DL. Within the GCS model description, the CL acts as a pseudocapacitor and thus presents a linearly changing electric potential from the surface (ϕ₀) to the Stern layer (ϕ_S, sometimes also referred to as the outer Helmholtz potential (65,66)), after which it decays exponentially in the DL due to screening.

Figure 3. SFG from charged interfaces showing (a) a schematic of the Gouy-Chapman-Stern model describing the electrostatics of the interfacial region, with the depth evolution of the electric field and potential as well as the two-layer regime, and (b) imaginary parts of the heterodyned SFG spectra of the pure air–water interface (right axis) and that covered with a negatively charged surfactant monolayer (dihexadecylphosphate, DHP, left axis), thus with no added subphase electrolyte, both recorded in the SSP polarization combination. The sharp positive contribution at ∼2320 cm^–1 is not a feature of the sample, but arises from the spectral referencing to quartz. Reprinted in part with permission from ref (24). Copyright 2024 American Chemical Society.

In the GCS model, however, the molecular nature of water is neglected, with its role being treated as a homogeneous dielectric medium. (48) The impact the molecular water interactions have on the electrostatic potential in the system is a complex problem and it is thus hard to evaluate the accuracy of this approximation. Nevertheless, the GCS model likely represents a reasonable mean field approach for describing the water response as any inherent anisotropy induced by the presence of the interface and any direct solvation to the charged headgroup or their counterions should be contained within the CL, being represented as by χ⁽²⁾. On the other hand, both the CL and DL can generate field-induced responses, which can be distinguished not only because they likely have different electrostatic regimes, but also different molecular structures (and thus potentially different susceptibilities). The overall effective susceptibility can therefore be written as in eq 12, including distinct χ⁽³⁾ tensors for the two layers.

χ_{eff}^{(2)} = L_{3} L_{2} L_{1} \int_{0}^{\infty} f_{3} (z) f_{2} (z) f_{1} (z) (χ_{CL}^{(2)} (z) - (χ_{CL}^{(3)} (z) + χ_{DL}^{(3)} (z)) \cdot \frac{d ϕ (z)}{d z}) e^{i Δ k_{z} z} d z

(12)

The integral in eq 12 can be split across the two distinct layers, separated at the Stern layer boundary, z_S, resulting in eq 13 where two simplifying assumptions have been made. First, as the DL structure can be assumed to be isotropic in the absence of the field, any depth dependence to χ_DL⁽³⁾ is neglected. Second, as the CL is typically much thinner than the coherence length, any propagation phase is very small and can thus be neglected in such a case (see Supporting Information).

χ_{eff}^{(2)} = \int_{0}^{z_{S}} χ_{CL}^{(2)} (z) - χ_{CL}^{(3)} (z) \cdot \frac{d ϕ (z)}{d z} d z - χ_{DL}^{(3)} \cdot \int_{z_{S}}^{\infty} \frac{d ϕ (z)}{d z} e^{i Δ k_{z} z} d z

(13)

While performing the integration over the CL contributions requires knowledge of the depth-dependency of the CL water structure (i.e., both susceptibilities) and thus cannot be generally performed, the integration over the DL can be evaluated just with knowledge of the depth-dependent potential. With the potential being described by eq 14 under the GCS model, integration by parts then yields eq 15 for the effective susceptibility, with the overall integrated CL contribution being written as an effective susceptibility from this region, χ_CL,eff⁽²⁾.

ϕ (z) = {\begin{array}{l} ϕ_{0} - \frac{(ϕ_{0} - ϕ_{S})}{z_{S}} z & 0 < z \leq z_{S} \\ ϕ_{S} e^{- (z - z_{S}) / z_{DL}} & z \geq z_{S} \end{array}

(14)

χ_{eff}^{(2)} = χ_{CL, eff}^{(2)} - χ_{DL}^{(3)} \cdot ({[ϕ (z) e^{i Δ k_{z} z}]}_{z_{S}}^{\infty} - i Δ k_{z} \int_{z_{S}}^{\infty} ϕ (z) e^{i Δ k_{z} z} d z) = χ_{CL, eff}^{(2)} - χ_{DL}^{(3)} \cdot (- ϕ_{S} e^{i Δ k_{z} z_{s}} - i Δ k_{z} ϕ_{S} e^{i Δ k_{z} z_{s}} \int_{0}^{\infty} e^{(- 1 / z_{DL} + i Δ k_{z}) z} d z) = χ_{CL, eff}^{(2)} + ϕ_{S} χ_{DL}^{(3)} e^{i Δ k_{z} z_{S}} \cdot (1 + \frac{i Δ k_{z}}{\frac{1}{z_{DL}} - i Δ k_{z}}) \approx χ_{CL, eff}^{(2)} + ϕ_{S} χ_{DL}^{(3)} \cdot (\frac{1}{1 - i Δ k_{z} z_{DL}}) = χ_{CL, eff}^{(2)} + \frac{1}{2} ϕ_{S} χ_{DL}^{(3)} \cdot (1 + e^{2 i atan (Δ k_{z} z_{DL})})

(15)

In the fourth line of eq 15, the propagation phase associated with the CL thickness, z_S, is once again neglected as it will generally make little impact due to the CL thickness being small compared to the coherence length (see Supporting Information for details). Importantly, eq 15 shows that the DL contribution contains potentially large phase-shifts depending on the Debye length, and is modulated by the Stern potential, ϕ_S, and not the surface potential, ϕ₀, as is frequently stated in various publications. (14,67−71) This distinction, having been also noted previously in passing, (29,54,66) naturally arises from the fact that the DL field does not start directly at the charged surface, but below the CL at the boundary to the DL. While this inaccuracy may seem trivial, the large potential drop across the CL makes this difference in fact substantial. Therefore, using the correct expression is crucial for an accurate electrochemical description of the interface and any thermodynamic quantity extracted from such measurements, for example surface charge density or local pK_a. (50)

Eq. 15 highlights the role that depth contributions play in the SFG spectra from charged interfaces. Specifically, it shows that the presence of the static field adds a second, potentially significant, contribution to the overall response arising from larger depths which could have a substantially different spectral line-shape for two reasons. First, the DL contribution arises from water in a different molecular environment compared to the CL, thus could show frequency shifts and bandwidth changes to the resonant features. Second, as the DL contribution is phase-shifted (originating from the propagation phase) by a potentially significant amount dependent on the Debye screening length (and thus the salinity of the system), the spectral line-shape will be distorted.

An example of these effects is given in Figure 3b which shows the SFG spectra for the pure air-D₂O interface (just as in Figure 2) as well as that covered with a monolayer of a negatively charged surfactant (dihexadecylphosphate, DHP). On first comparison, the two spectra appear substantially different. Unlike the pure air–water interface, the surfactant-covered interface shows strong, sharp resonances between 2800 and 3000 cm^–1 which predominantly arise from the terminal CH₃ groups of the well-packed surfactant monolayer. (20) Beyond this, however, the water response (in the lower frequency range, <2700 cm^–1) also appears notably different. Specifically, the OD resonances appear highly red-shifted, falling at ∼2400 cm^–1 compared to ∼2550 cm^–1 for pure air–water, significantly larger in amplitude, and entirely positive (unlike pure air–water which shows significant positive and negative resonances). These observations align well with expectations at the charged interface as the solvation of the charged surfactant head-groups and DL contribution will present greater overall H-bonding compared to the hydrophobic air interface, lowering the overall resonant frequency. (72−74) Furthermore, the static field will cause significant orientational alignment of water, thus boosting the amplitude, and the alignment with the field from the negatively charged interface will be such that the molecular dipoles are pointing “up”, yielding only positive resonances. Finally, and importantly, while the pure air–water spectrum will show no noticeable phase distortion as it arises from an ultrathin interfacial layer, the spectrum from the charged interface clearly shows a broad negative feature above the main (positive) resonance, between 2600 and 2800 cm^–1. This dip-like feature is a clear manifestation of a depth-related phase distortion, with some of the dispersive (real) line-shape entering into the imaginary part. Overall, this demonstrates that the large Debye length associated with having low salinity conditions in this sample leads to significant effects on the line-shape of the overall response.

2.5. Entanglement with Experimental Parameters

From the above discussion on a specific class of sample system where depth is highly relevant, we now turn to a fundamental consequence of the integrated nature of the effective susceptibility. As shown in eq 3, χ_eff⁽²⁾ is a function of the coherence length, which is a parameter that depends on the experimental settings such as the specific beam geometry and input frequencies of the probing fields. As such, two measurements with varying coherence lengths can yield very different spectra. This dependency of χ_eff⁽²⁾ on Δk_z cannot simply be removed since the terms containing the coherence length are functions of the integration variable so they cannot be moved outside the integral and therefore their values directly influence the line-shape of the obtained spectra. Hence, the depth-dependent structure and the experimental settings are entangled in the observed response. However, the impact of this entanglement is highly variable.

In the case that the interfacial signals originate entirely from a negligibly thick layer (which could be approximated by a delta function, category (i) above), there is no significant propagation phase to be considered and the sensitivity to experimental parameters would almost completely vanish on the assumption of minimal dispersion for the nonresonant fields. In such cases, the χ_eff⁽²⁾ spectra (when considering only a single contributing tensor component) obtained from measurements with varying experimental settings should be identical, with examples being OTS on fused silica or even the resonant O–D stretching response of the air–water interface where only ∼2–3 molecular layers form the anisotropic structure. (75) The added propagation phase for such a thickness is negligible for typical values of the coherence length (e.g., for z < 1 nm and

\frac{1}{Δ k_{z}} \sim 50 nm

, the added propagation phase is contained with ∼1°). Therefore, the resulting SFG spectra here show minute phase distortions and the signals from each layer are essentially purely additive.

However, the moment where nanoscale depth contributions are present (e.g., for water at charged interfaces), the spectra from measurements employing different experimental settings will significantly deviate due to the modulation of the signals from different depths by propagation phase-shifts and local field corrections. In this case, it is impossible to remove the impact of the experimental parameters and report an unbiased single spectrum that could easily be compared to other measurements with different experimental settings (frequencies, geometry, etc.). The only characteristic, unbiased “spectrum” would be the experimentally inaccessible full two-dimensional representation, χ⁽²⁾(z), which can typically only be obtained from simulations. As shown in section 4, however, advanced methods can recover some of this information by modulating contributions from different depths.

3. Complications for Structural Interpretations and Molecular Recognition

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In the previous section, we emphasized the difference between the idealized measured quantity, the 2D second-order susceptibility, and the true measured quantity in experimental SFG investigations, namely the spatially integrated signal. In doing so, we highlighted the critical role that depth plays in the response, entangling the material properties with experimental parameters and leading to potentially significant line-shape distortions. In this section, we will now turn to an examination of the challenges for identifying specific molecular motifs and obtaining structural information from this measured quantity. Specifically, we discuss the impact of possible line-shape distortions that can originate from the depth-integrated nature of the effective second-order susceptibility (as mentioned above), but also from the interference from multiple sources, including the vibrationally resonant response with other nonlinear signal contributions. These cases are each individually addressed in the following and their impact is demonstrated using specific examples.

3.1. Depth Related Line-Shape Distortions

The interpretation of spectral features within a measured SFG spectrum can easily be misled by the presence of phase distortions. As phase-shifts lead to mixing between real (dispersive) and imaginary parts (absorptive) of the vibrational line-shapes, single resonance peaks can appear as doublets (dip-peak features) in a measured spectrum. As an example, we refer to the spectrum of charged water interfaces in Figure 3b. The spectrum could easily be evaluated without considering this effect, concluding that the negative feature between 2600 and 2800 cm^–1 arises from a weakly H-bonded water environment with pointing “down” molecular dipoles. This conclusion, however, contradicts expectations given that the negatively charged interface leads to a strong preference for pointing “up” molecules, and pointing “down” molecules would not be expected to have such weak H-bonding. (69,76) In reality, this negative feature simply originates from the dispersive (real) part of the intrinsic sample response that enters the measured imaginary spectrum through depth-induced phase-shifts. This example clearly demonstrates that a correct interpretation of the observed spectral features requires at least some knowledge on the length-scale on which the nonlinear signal contributions decay as a function of depth.

3.2. Interference between Multiple Contributions

Many sample systems of interest contain multiple SFG contributions that can have very different spatial origins, e.g. CL and DL from charged aqueous interfaces, as in the example discussed above. The presence of such multiple contributions can also lead to major challenges in the data analysis. Within the example above (charged aqueous interfaces), it is important to point out that even with information on the depth evolution in hand, it is not possible to simply phase-correct the obtained overall SFG spectra and evaluate the unperturbed spectrum. As shown in eq 15, the overall SFG response from such an interface is composed of two contributions, one from the CL and a second contribution from the DL. As only the second contribution contains significant phase-shifts, the only way to remove the effect of depth induced phase distortions is to first correctly decompose the overall spectrum into CL and DL contributions and subsequently only phase-correct the DL spectrum. Such a decomposition clearly cannot be done based on a single SFG measurement.

In their recent work, however, Gibbs and co-workers showed that, by combining phase-resolved SFG spectra with heterodyned, off-resonant second harmonic generation (SHG) and streaming current measurements, the two components (CL and DL) can be isolated. (65,77) Furthermore, by using the maximum entropy method (MEM) of obtaining complex SFG spectra from homodyned experiments, they also proposed that the depth information is entirely contained within the associated error phase that generally complicates such analysis, therefore leading to interesting possibilities for depth-related studies. Equally, other methods which combine multiple SFG measurements from systems with altered subphase concentrations have also been proposed as an approach to separate the CL and DL contributions from charged interfaces. (70) While these methods do indeed represent progress in the separation of these multiple depth-dependent signals, it is important to note that they are not without crucial assumptions. These include, for example, equating the Stern potential that modulates the DL contribution to either the surface or zeta potentials, or that the CL contribution is unvarying with changing subphase salt concentrations. While these may well be good approximations for very specific cases, they are not generally applicable. Nevertheless, as shown later, experimental advancements in SFG spectroscopy have now made these decompositions possible without the need for such assumptions.

Another source of interference between multiple SFG contributions is that between the vibrationally resonant and nonresonant responses. These contributions are always present and can lead to difficulties in data analysis and interpretation, even for the case where there is no extended anisotropic depth, such as the air–water interface. Unlike the resonant (R) contribution which is dominated by interactions with the vibrational part of the molecular wave function and thus the asymmetry in the Morse potential, the NR contributions are dominated by higher lying states and are therefore sensitive to the asymmetry in the polarizable electron cloud. (78) In consequence, the two contributions probe vastly different aspects of the molecular structure and can thus potentially originate from different spatial regions within the interface. Additionally, the two contributions will have different spectral line-shapes, with the former vibrationally resonant part presenting strong amplitude and phase modulations as a function of frequency (due to the resonances), but the NR contribution being largely independent of the IR frequency. In phase-resolved (heterodyned) spectra the output of multiple contributions is purely a linear-superposition. Therefore, in the case of the NR contribution, it simply adds a constant offset to the spectra, as shown in eq 16. This is demonstrated in Figure 4a for the air–water (D₂O) interface which shows the real and imaginary parts of the phase-resolved spectrum. Note that, the NR contribution can clearly be seen to predominantly result in a constant negative offset in the real part.

S_{heterodyne} \propto χ_{eff}^{(2)} = χ_{R}^{(2)} + χ_{NR}^{(2)}

(16)

Figure 4. SFG spectra of the air–D₂O interface recorded in the SSP polarization combination showing (a) phase-resolved response split into real and imaginary parts, highlighting the significant nonresonant contribution (NR), and (b) intensity spectrum for the overall response (resonant + nonresonant) and the resonant-only response, having subtracted the H₂O NR spectrum. Reprinted in part with permission from ref (24). Copyright 2024 American Chemical Society.

In such phase-resolved SFG measurements, the presence of the NR contributions obviously does not affect peak positions and line-shapes of the resonant responses. The situation is, however, fundamentally different for intensity (homodyned) SFG measurements, which continue to be prevalent in SFG research. (79−87) The output signal is then related to the square of the effective susceptibility, as shown in eq 17, meaning the R and NR contributions interfere and can significantly alter the observed line-shape.

S_{homodyne} \propto {| χ_{eff}^{(2)} |}^{2} = {| χ_{R}^{(2)} |}^{2} + {| χ_{NR}^{(2)} |}^{2} + 2 | χ_{R}^{(2)} χ_{NR}^{(2) *} |

(17)

While in some cases this distortion effect may well be negligible, in many prominent cases the NR contribution is significant, leading to complete deformation of the vibrational spectrum. One example of this is clearly shown in Figure 4b which presents the intensity spectrum of the overall response from the air-D₂O interface, which has become the well-known showcase spectrum of the SFG technique, (10,88−90) along with the same but having removed the NR contribution through subtraction of the air-H₂O spectrum, i.e., obtained through isotopic substitution measurements (green trace). (24,91) This comparison shows that the line-shape and peak positions of the SFG intensity spectrum are drastically affected by the interference of the R part with the NR contribution, making it in fact a poor and misleading representation of the vibrational spectrum of the interface without correction for the NR background. The resulting spectral distortions are meanwhile quite comparable to the distortions originating from the phase-shifts discussed above. As the NR contribution is predominantly located in the real part of the response, the interference makes some part of the dispersive resonant line-shape enter the intensity spectrum. If only such an overall intensity spectrum is obtained from a measurement, the interference effect can only be removed by correctly decomposing the spectrum into its R and NR contributions by performing a multiparameter fit. To obtain reliable results from such a fit, however, significant prior knowledge of the center-frequencies and spectral shapes of the different resonant features is required. For a complex system such as the air–water interface, such knowledge is generally not available. The impact of the NR contribution on the resonant line-shape has therefore often been neglected, which has e.g. resulted in the misled initial assessments of the air–water interface spectrum, where the large red-shifted peak (here at ∼2500 cm^–1 for D₂O) was interpreted as being indicative of significant “ice-like” water that is strongly H-bonded. (88,92) More recently, the strong interference effect with the NR contribution and the resulting spectral distortions has been noted, (10,93) and this structural interpretation was shown to be inaccurate, with the main H-bonded resonance being substantially higher in frequency, more in-line with the expectation of liquid water.

From the discussion above, it is clear that the analysis of SFG spectra can become highly demanding whenever multiple contributions are present. This is particularly true for homodyned intensity SFG measurements where no phase resolution is obtained. In the case of complex sample systems including multiple depth contributions, such as charged aqueous interfaces, it is therefore evident that phase-resolved (heterodyned) SFG has substantial benefits over conventional intensity approaches.

3.3. Quadrupolar Contributions

In all of the discussion above, just as with the vast majority of SFG research, the output signals are assumed to arise from structurally anisotropic regions, i.e., from a thin region at the interface (for most media such as water). This assumption thus only considers the purely electric dipole-driven transitions (EDA) and neglects any higher order spatial contributions. Beyond this approximation, however, second-order signals can also arise from electric quadrupolar-driven transitions which, despite their generally lower transition probabilities, can become significant as they are not restricted to structurally anisotropic regions. (7,21) This means that they are not subject to the cancellation of signals from oppositely oriented molecules and can thus, in principle, yield signals from in-plane molecules and even throughout the entire bulk. Unlike the dipolar contribution which arises from the coupling of the electric fields of the two incident fields, the electric quadrupolar contributions source from the coupling between a field and a field gradient. (7,21,94) In this sense, there are two aspects of the electric fields that lead to field gradients which can yield quadrupolar signals. First, there is the sharp change in field amplitude that occurs at the interface due to the anisotropy in the dielectric function (i.e., the gradient in the local field factors). This contribution is hence tightly bound to the interface, with a depth profile of only a few angstroms, and is thus named the interfacial or anisotropic quadrupolar signal (IQ). While this signal only arises from a few molecular layers, it could become significant due to the sheer magnitude of the field gradient at the interface. Second, there is the field gradient arising from the oscillatory nature of the field, which clearly persists throughout the entire medium, yielding a bulk or isotropic quadrupolar contribution (QB). A schematic of the source mechanisms and different spatial origins of the different contributions is illustrated in Figure 5.

Figure 5. Schematic showing the spatial and mechanistic origins of the electric dipolar and quadrupolar signals to the overall SFG response. ID: interfacial dipole, IQ: interfacial quadrupole, QB: bulk quadrupole. (7)

The presence of either contribution interfering with the purely dipolar (structurally anisotropic) signal complicates the analysis for several reasons. First, the quadrupolar contributions are governed by rank-4 tensors (due to the field gradient direction coupling into χ⁽²⁾) so, as mentioned above, the signs of the output signals are independent of the molecular orientation, unlike dipolar signals. Therefore, the quadrupolar spectrum arising from the same molecular environments will generally yield a different line-shape than the dipolar spectrum. Second, as demonstrated schematically in Figure 5, the three contributions generally arise from different spatial regions, and thus likely probe different molecular structures and environments. This will hence further alter their relative line-shapes. Finally, due to their different spatial profiles and mechanistic origins, the apparent depth-related phase distortions will be different. Specifically, as the IQ contribution is tightly bound to the interface, it will experience a very minimal phase distortion due to propagation effects, with the dipolar signal likely arising from a thicker region. The QB contribution in contrast contains a 90° propagation phase-shift due to its bulk origin. The intrinsic QB response is, however, also phase-shifted by 90° due to the coupling to the oscillatory component of the field gradient which leads to an overall 180° phase shift, i.e., manifesting purely as a sign-flip in the local response. (7,24) It will therefore show similar behavior as a purely interfacial signal from electric dipolar sources, with no mixing between real and imaginary parts. This property makes it in turn very challenging to distinguish the two contributions (interfacial and bulk) and to extract the typically desired interfacial response of the system. The most characteristic difference between the dipolar and the QB responses is their different spatial origin which further underlines the necessity to obtain depth information on the SFG signal sources.

As many structural investigations of interfaces rely on the interpretation and comparison of experimental SFG spectra to the analogous calculations from simulations, it is also important to mention that many simulations, including those yielding the results shown in Figure 1, typically do not fully address the quadrupolar contributions. This hence limits the possible insights into the structural motifs present at the interface that can be obtained from quantitative or even qualitative comparison to calculated SFG spectra. For such comparisons, it is clearly crucial to include all potential contributions to the observed signals.

The analysis of SFG spectra in terms of quadrupolar signal contributions is further hampered by the fact that the size of the quadrupolar moments can highly vary with infrared frequency. This means different vibrational modes as well as the NR contribution may have largely different weightings between the dipolar and different quadrupolar signals. For aqueous systems, this variability across different spectral transitions seems to be substantial, with the NR contribution having been previously suggested from both experimental measurements and accompanying theory to be dominated by quadrupolar contributions, with a particularly significant role from the IQ mechanism, (95,96) and more recently being experimentally shown to have also have a significant QB contribution. (24) By contrast, the OH stretching mode of water (R contribution) seems to mainly arise from dipolar signals owing to its blue-shifted frequencies being indicative of a weaker H-bonding environment (as expected for the interface), as well as the observed sign-flip for oppositely oriented molecules. This too differs from the resonant H–O–H bending mode which, like the NR contribution, also appears to be dominated by quadrupolar signals. (97) Such observations highlight that the role of quadrupolar contributions in SFG investigations, particularly for aqueous interfaces, cannot be simply neglected, and must be treated separately for each feature of the overall response.

4. Recent Developments in Probing Aqueous Interfaces

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In the discussion above, various challenges associated with SFG measurements have been identified. A key point within this discussion is the role of signal contributions from extended sample depths as well as the necessity to acquire such depth information for a correct interpretation of SFG spectra and to obtain detailed understanding of interfacial molecular structures. Additionally, the importance of separating different signal contributions such as R and NR responses was emphasized. Recently, several advancements in SFG spectroscopy have been developed to address these challenges and we review and discuss a selection of the most promising approaches among these developments.

One very crucial technical advancement and milestone in SFG spectroscopy already dates back more than two decades, namely the advent of phase-sensitive measurements. (9,98−101) The clear advantage of this technique is twofold: first, it doubles the information content of measured SFG spectra by revealing amplitude and phase information, and second, it makes all different signal contributions become linearly superimposed in the final spectra without the rise of any complex interference terms between them. This linearity highly simplifies the data analysis and allows for performing advanced measurements protocols such as difference spectroscopy in a straightforward manner. (24,102) Like for all vibrational spectroscopy techniques, isotope labeling approaches are of particular importance in this context because this enables the unambiguous identification of molecular species and the decomposition of congested spectra into specific vibrational modes. (103,104) Furthermore, comparison between vibrational spectra of different isotopologues also allows for the precise decomposition of SFG spectra into the vibrationally resonant and nonresonant parts, as demonstrated with the example of the air–water interface in Figure 4 and in reference. (24,91) It should be noted that the additional phase information obtained in phase-sensitive measurements in principle also already contains the desired depth information which is encoded in the propagation phase. However, from such measurements alone, the propagation phase cannot be isolated as it is convoluted with the intrinsic spectral phase of the vibrational resonances. (36,37) For the determination of the propagation phase, additional information from experimental measurements or from theory is therefore required.

One powerful experimental concept to directly obtain depth information from SFG measurements is the modulation of the coherence length. While the entangled nature of the material properties with experimental parameters due to signal contributions from extended sample depths is, on the one hand, not ideal for isolating the structural properties from the interface, it does, on the other hand, provide a useful route for accessing the depth dependency of the susceptibility, and thus the depth-dependent structure. As discussed earlier, signal contributions from below the interface contain the propagation phase factor, e^iΔk_zz which, after integration in z, leads to phase-shifts and amplitude changes in the observed spectrum. (36) An illustration of the evolution of spectral amplitude and propagation phase with increasing anisotropic depth is given in Figure 6a for two selected values of Δk_z. From this plot two things become apparent: first, both observables clearly vary with the anisotropic depth, as expected, and second, they also both highly depend on the value of Δk_z. Therefore, by modulating the coherence length, 1/Δk_z, the depth information becomes in principle separable from the intrinsic (local) susceptibility. Obviously, for the best sensitivity of this method, the modulations should be as large as possible. The scale of the differences in these two observables are depicted in Figure 6b which shows the resulting amplitude ratio (black) and phase difference (red) as a function of anisotropic depth for the two selected values of the coherence length (see Supporting Information for more details). While these plots are clearly not generally applicable to all experimental settings and sample systems, they nonetheless give an idea of the expected modulations that would be observed for an interface with a given anisotropic depth.

Figure 6. Amplitude and phase variations of the observed SFG signal due to depth. (a) Plots of the amplitude scaling and propagation phase shift for two different coherence lengths that are typical for SFG studies of aqueous interfaces. While the amplitude prefactor is only shown for two wavevector mismatches, as it independent of sign, the propagation phase is presented with Δk_z(2) taking either a positive or negative value, indicated by the two labels Δk_z(2+) and Δk_z(2−). (b) Resulting amplitude ratios and phase differences between the observed signals with these coherence lengths, showing the different cases for equal and opposite signs for the two Δk_z values, i.e., ϕ(1) – ϕ(2+) and ϕ(1) – ϕ(2−), respectively, for the phase differences (with the amplitude ratio being independent of sign). Further details on these plots can be found in the Supporting Information.

Interestingly, while the amplitude scales with the modulus of the coherence length, the resulting phase-shift scales with Δk_z (inverse of coherence length), including its sign. Therefore, two different modulation regimes exist for the phase, which are each presented in Figure 6b: one with the same signs of the two Δk_z values (shown in solid red), and one with opposite signs (shown in dashed red). The three presented plots in Figure 6b thus highlight two important points. First, the phase difference and amplitude ratio show different functional forms. Therefore, the information contained in both are complementary and not degenerate. In consequence, while the depth information can clearly be extracted from either observable for the case where the response can be described simply by a single depth-related parameter, e.g. an exponential decay function, for more complex systems that involve multiple depth-dependent contributions, one observable is generally insufficient. In this context, it is important to note that most systems of interest, e.g. charged interfaces, multiple contributions are present. Therefore, combining both pieces of information allows for a more accurate characterization of the depth-dependent χ⁽²⁾(z), for example, the unambiguous identification of any bulk signals with quadrupolar origin. (24) The second important point highlighted by Figure 6b is associated with the vastly contrasting phase difference plots for the equal and opposite sign wavevector mismatches. This shows that having opposite signs yields overall much higher depth sensitivity in the phase difference, gives sensitivity to both small and large depths, and importantly presents a monotonic function which yields a clear 1-to-1 relationship to the anisotropic depth. Nevertheless, while having opposite signs for Δk_z is clearly favorable, this is not generally the case for techniques exploiting the coherence length modulation, as discussed below.

4.1. Transmission/Reflection SFG

Modulation of the coherence length was first highlighted by Shen et al., who discussed the principle of measuring the SFG intensity in both reflection and transmission to determine the isotropic quadrupolar contributions that arise throughout the entire bulk. (105) As the output SFG wavevector has opposite signs for the z-components in reflection and transmission, such measurements represent a substantial modulation of the coherence length of typically 3 orders of magnitude (but Δk_z stays generally positive). This hence leads to a large amplitude modulation in the spectra for any bulk contributions, which allowed the authors to completely disentangle them from the surface SFG signal of the benzene-air interface. Clearly, this approach can be very powerful for separating interfacial and bulk contributions (e.g., dipolar vs quadrupolar) and, although it has not been demonstrated yet, also has significant potential for obtaining depth-dependent structural information from the interface. Equally, while it has only been performed using intensity measurements (i.e., only using the amplitude information), extracting the phase information would also lead to greater structural insight, as mentioned above. Recording both the reflection and transmission responses has, however, not become a widely adopted experimental approach due to its associated significant experimental challenges, particularly with obtaining accurate phase information from both signals. Additionally, the large coherence length obtained for the transmission signal leads to the necessity to account for additional effects such as pump beam depletion which makes the data analysis more complex.

4.2. Variable-Angle SFG

While the principle of modulating the coherence length for obtaining depth-dependent structural information at charged interfaces was first mentioned by Shen and Tian, (54) its was not until recently that it was experimentally demonstrated. Three alternative approaches have been presented, with each being used to study the depth-dependent structure at aqueous interfaces. Hore and co-workers showed the initial approach where the incident angle of the input beams is altered, thus modulating the specific z-component of their wavevectors. (63,106,107) This can be done in full reflection geometry and hence circumvents the challenge of detecting the transmitted beam. Furthermore, by using an internal reflection geometry that includes the critical angle, the authors showed that the coherence length can be significantly modulated, which is not the case for regular reflection geometries. This hence allows for high depth accuracy, especially as the variable-angle (VA-SFG) approach can generate many unique data points. The authors demonstrated this technique by successfully separating the CL and DL spectra from such angular-dependent intensity measurements and compared the water structure in the corresponding regions, clearly highlighting the potential of this method. The challenges associated with this technique are that it requires multiple subsequent measurements rather than simply recording multiple responses from a single experiment, and involves an elaborate sample geometry. Additionally, the internal reflection approach restricts the classes of interfaces that can be investigated, and proximity to the critical angle means the output signals become very sensitive to local field effects which must be correctly accounted for. Finally, obtaining accurate phase information with this technique would also be analytically challenging due to the strong angular-dependence on the reflection phase of any reference.

4.3. Frequency (Momentum)-Dependent SFG

As the second approach, Hsiao et al. performed momentum-dependent SFG (MD-SFG) whereby they modulated the frequency of the visible upconversion beam, and thus the magnitude of its wavevector. (29) That way, the depth information also becomes accessible, which was demonstrated by extracting the CL spectrum from the overall water response at charged interfaces using the combination of intensity and phase-resolved spectra. The distinct benefit of this approach is that it does not require a complex sample setup (like the variable-angle approach), and also is not restricted to certain sample classes. Furthermore, just as the variable-angle technique, it provides the opportunity to record several unique data points to extract the depth information. In principle, this approach could also include the analysis of the phase changes due to depth to further increase the analytical capabilities, although this has not been demonstrated. Nevertheless, multiple separate measurements are again required and, as this technique involves altering the frequencies, it is subject to potential dispersion effects which can make the spectral corrections and analysis more challenging. While for relatively small changes in frequency these are minor effects, this is counter to achieving large coherence length modulations, meaning a compromise must be made. Finally, in contrast to the other methods, MD-SFG cannot effectively separate interfacial and bulk signals since the bulk quadrupolar response is highly insensitive to upconversion frequency. (24)

4.4. Combining Sum- and Difference Frequency Generation (SFG/DFG)

The third approach for extracting depth information from the modulation of the coherence length is a recent development combining SFG measurements with their corresponding DFG responses. (24,36,37) For probing the same resonant vibrational information, the SFG and DFG responses not only have different magnitude wavevector mismatches, but they also differ in sign. The different magnitudes (SFG and DFG signals differ in frequency) is analogous to the momentum-dependent approach, and therefore can obtain similar insight from the amplitude modulations. Beyond this, however, the different signs of the two wavevector mismatches is a unique feature of this approach and makes the phase difference a far more sensitive parameter for depth-dependent studies (as shown above). As a result, SFG/DFG can obtain exceptional depth accuracy, with phase-resolved studies having been shown to achieve sub-nanometer resolution on model systems of self-assembled monolayers on quartz, (36) as well as being used to extract the first experimental measurement of the anisotropic “healing depth” of the air–water interface. (24) The SFG/DFG method also benefits from the fact that both responses are simultaneously generated, meaning the depth information can be extracted from a single measurement, in contrast to the two aforementioned techniques. Furthermore, as bulk quadrupolar contributions have different amplitudes in SFG and DFG pathways, the bulk signals can also be separated from the interfacial response and their mechanistic origin (dipolar/quadrupolar) identified. (24) One particular additional benefit of SFG/DFG that contrasts to the MD-SFG or VA-SFG, is that other phase distortions to the spectra arising from complex high frequency Fresnel factors can also be completely removed as they too modulate the SFG and DFG responses differently. (37) While this may not be especially relevant to many aqueous interfaces, it is crucial whenever they are in the presence of media which absorb in the visible frequencies, such as metals. This hence makes it particularly important for electrochemical systems.

An example of the effectiveness of the SFG/DFG method is shown in Figure 7 which presents both the real and imaginary parts of the heterodyned SFG and DFG spectra in the O–H stretching region for the air-H₂O interface covered with a negatively charged surfactant, dihexadecylphosphate (DHP). This is shown for two subphase salt concentrations, 0.1 M (Figure 7a) and 10^–4 M (Figure 7b), which have substantially different Debye screening lengths, namely 1 and 30 nm, respectively. Since the SFG and DFG responses probe the same susceptibility, they have the same intrinsic line-shapes, only that they are phase-shifted in opposite directions owing to the different signs of their wavevector mismatches. By representing the responses schematically in phase diagrams (Argand plots) at four different frequencies through the O–H stretching region, Figure 7 demonstrates that the phase difference between SFG and DFG is both maintained, and evidently scales with the screening length, as expected. This clearly highlights that important depth information about the interfacial region can be separated and extracted from the two spectra, including the separation of the CL and DL contributions, just as for MD-SFG and VA-SFG.

Figure 7. Heterodyned SFG and DFG spectra recorded in the SSP polarization combination from the air–water interface covered with negatively charged surfactants (DHP), showing both the real and imaginary parts for a subphase NaCl salt concentration of (a) 10^–1 M and (b) 10^–4 M. Also shown are schematic Argand plots of the SFG and DFG phases at selected frequencies through the resonant line shape.

While there are clearly many benefits for SFG/DFG, it too comes with challenges. First, and perhaps the most important of which is that, in order to extract the full benefits of the technique by measuring not only the amplitudes but also the phases of the two nonlinear responses, exceptional accuracy in both the absolute phase and exact frequencies of both responses is required. This hence makes for an extraordinary experimental challenge and means the technique cannot feasibly be implemented into any standard SFG spectrometer, but instead requires a more specialized setup. Beyond this, due to the different output frequencies of SFG and DFG, it too (just like the MD-SFG) is subject to potential dispersion effects. With the given frequency differences between SFG and DFG, however, these effects are usually negligible and, if not, can be effectively removed by recording SFG and DFG with the same three frequencies in separate measurements (although this procedure loses the above-mentioned simultaneity benefit). (24,37)

4.5. Summary of Coherence Length Modulation Techniques

While the four techniques presented above follow the same principle (modulation of the coherence length/wavevector mismatch), the resulting spectral modulations in phase and amplitude of the measured spectra can be distinctly different. A general evaluation of their efficacy in extracting depth information is not straightforward as the achieved modulations highly dependent on both the specific properties of the sample systems and realistic experimental limitations. To give some comparative indication of their performance, we turn to the example of charged aqueous interfaces due to their widespread relevance in such studies. In Figure 8 we compare the percentage amplitude modulations and absolute phase differences for the different techniques, using values for their wavevector mismatches taken from the literature. (24,29,63) This comparison is shown for diffuse layer contributions with two different Debye screening lengths, Z_DL, 10 and 100 nm. These modulations are calculated based on the differences in amplitudes and phases of the two measured responses for an interface with the specified depth i.e., the difference in amplitude and phase between the transmitted and reflected SFG responses from such an interface, or the differences between SFG for two different incidence angles, etc. (see Supporting Information for more details). The striped red boxes represent the theoretical phase differences as they have not been reported due to the lack of corresponding phase-resolved measurements for these techniques.

Figure 8. Comparison of the amplitude (black) and phase (red) modulations of the different methods for accessing depth information from modulating the coherence length. The values for the plots are shown for two different anisotropic depths that are typical values for charged interfaces, with Debye lengths of 10 and 100 nm which occur for electrolyte concentrations of 1 mM and 10 μM, respectively. The plots are based on calculations described in the Supporting Information using values for the coherence lengths from the literature. (24,29,63) The red striped bars indicate the potential phase differences using these literature values; however such phase-resolved measurements having not been demonstrated. T/R: transmission/reflection, VA: variable-angle, MD: momentum (frequency)-dependent.

From the plots in Figure 8, it becomes clear that for the selected examples, two techniques particularly stand out, namely the T/R-SFG and combined SFG/DFG methods. It is important to mention again that these evaluations only refer to the selected examples. For the typically more complex systems, such analysis could lead to different weightings, so the optimum technique for a specific investigation can only be evaluated on a case-by-case basis, especially considering the unique experimental challenges and restrictions associated with each method.

4.6. Other Methods

Beside the coherence length modulation methods, there have been alternative ways presented of obtaining depth information. However, these either include various degrees of theoretical simulations and models, or only probe isolated aspects of the depth-dependent structure.

Recently Nagata et al. showcased a technique exploiting polarization-dependent measurements to distinguish between resonances which source from different dielectric environments. (108,109) Specifically, the z-dependence of the dielectric function is contained within the Fresnel corrections for the ZZZ contribution within the overall PPP polarization response. Therefore, by measuring both PPP and SSP polarization combinations, this contribution can in principle be isolated and, by comparison to a depth-dependent model of the dielectric environment at the interface, the depth origin of the signal can be extracted. Given that this method utilizes the variation in dielectric function across the interface, it is typically only sensitive to depth changes across the first nanometer. Using their method, the authors were therefore able to detect a height shift of ∼0.9 nm for formic acid monolayers upon increasing surface concentration. (108) One challenge with this method, however, is that it is clearly sensitive to the specific functional form of the dielectric profile across the interface and therefore replies on the accuracy of the theoretical predictions of this. As this polarization-dependent method accesses an entirely different range of structural properties at the interface, being sensitive to Ångström-scale depths rather than ∼1–100 nm for modulation of the coherence length, the two techniques could nevertheless be used simultaneously to provide complementary information about the depth-dependency of the susceptibility and gain far greater elucidation of interfacial structure.

Another recent and compelling development combines SFG with IR pump pulses, either in a traditional pump–probe scheme to record the time-dependent vibrational relaxation of the interface (TR-SFG), or in a two-dimensional spectroscopy scheme (2D-SFG) to assess intermode couplings and energy transfer mechanisms. While these techniques do not get the same level of direct depth elucidation as those discussed above, they do however gain access to enhanced structural insight through the coupling and relaxation mechanisms of the different structural motifs at the interface. By comparing these to known quantities from the bulk, therefore, some indication about the onset of bulk properties can be gained. For example, time-resolved IR pump–SFG probe measurements were utilized by the groups of Ye and Tahara to probe the T₁ vibrational relaxation time of the O–H stretching modes. (110,111) In doing so, Tahara et al. showed that the hydrogen-bonded water at the air–water interface showed strong similarities with bulk-like water, suggesting a fast onset of bulk-like behavior. By contrast, Ye et al. studied water at charged interfaces, where they used the dependency of the relaxation time on number of accessible intermolecular states to show that it can be used to assess the thickness of the CL for varying salt concentrations and species present at the interface. These indirect assessments of the depth dependency thus provide a useful alternative way of assessing the higher dimensionality of the interfacial water structure.

It is also worth noting that Benderskii et al. used an alternative approach to assess the onset of bulk properties at the air–water interface whereby they studied the vibrational coupling mechanisms of the “free” OH stretch using isotopic dilution SFG measurements. (31) In doing so, they showed that these structural motifs that only exist very close to the interface are coupled solely to the other OH group on the same water molecules (intramolecular coupling). By then analyzing the stretching frequency of this coupled mode, they also showed that it displayed almost bulk-like properties. From this, just as Tahara et al. did using pump–probe SFG measurements, (111) they concluded that the onset of bulk-like properties is remarkably fast.

5. Future Directions

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Over the past four decades, since its initial demonstration, vibrational SFG spectroscopy has proven to be an invaluable tool for studying interfacial structure. Within this time frame, there have been several important development steps which have brought forward its possibilities and improved the analysis of interfacial systems, but one key advancement which truly slingshot the capabilities of SFG was the introduction of phase-sensitive heterodyned detection. Because of its substantial improvement in structural insight and the fact that the first phase-resolved spectrometers have become commercially available, we expect that phase-resolved measurements will soon become the dominant SFG method.

Beyond this, the need of obtaining detailed depth information as well as a more thorough characterization of the different SFG signal contributions has become increasingly visible in the scientific community and will certainly continue to be a crucial topic in future studies. Therefore, with the techniques discussed above precisely addressing these questions, it is once again clear that a distinct and substantial improvement in the capabilities of SFG has been made. Using these methods now allows us to not only characterize the molecular constituents present at an interface along with their orientations and intermolecular connectivities, but also gain access to specific details about the depth-dependent structural anisotropy. Given that depth is an intrinsic and often important property of many interfaces, we highly anticipate these approaches to represent another significant milestone in the evolution of interfacial spectroscopy studies.

While all of the recent developments obtain similar information on depth, their sensitivities for individual aspects of the interfacial structure differ, and they each have their particular challenges, for example the applicability to only certain sample classes, sensitivity to theoretical models, or experimental complexity. While some of these challenges may well be overcome by additional development steps in the future, it is important to note that the greatest gain in insight likely lies with certain combinations of them, e.g. polarization-dependent measurements with coherence length modulation, or IR-pump-SFG/DFG-probe measurements.

One particular research field where the above techniques will be especially influential is the investigation of the molecular structure at aqueous interfaces. In these sample systems, nanoscale depth is often a significant and deterministic parameter of the interfacial properties, e.g., for charged interfaces where the existence of an electric double layer (EDL) creates a substantial variation in properties with depth. With these methodologies, the signals from aqueous interfaces can be better understood in terms of the decay lengths of their anisotropic signals, contributions from different electrostatic regimes (e.g., separating CL and DL spectra), and even isolating the purely interfacial signal from any isotropic quadrupolar contributions. Overall, this will allow for comprehensive tests of models such as the GCS model and potentially its development into more sophisticated forms which directly account for, and accurately describe the molecular response of the constituent water. Finally, the ability of the SFG/DFG method to isolate the purely resonant signals from the impact of both depth and added phases from the complex Fresnel factors shows significant promise for the study of electrochemical systems which have thus far proven to be exceptionally challenging.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.4c06650.

Materials and methods used in this work, discussion on the approximations in the SFG response from charged interfaces, effect due to modulating the coherence length (PDF)

jp4c06650_si_001.pdf (225.92 kb)

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Author Information

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Corresponding Authors
- Alexander P. Fellows - Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany; https://orcid.org/0000-0002-5885-8144; Email: [email protected]
- Martin Thämer - Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany; https://orcid.org/0000-0002-9631-9280; Email: [email protected]
Authors
- Álvaro Díaz Duque - Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
- Vasileios Balos - Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA Nanociencia), 28049 Madrid, Spain; https://orcid.org/0000-0001-7606-6653
- Louis Lehmann - Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany
- Roland R. Netz - Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany; https://orcid.org/0000-0003-0147-0162
- Martin Wolf - Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany; https://orcid.org/0000-0001-7226-9592
Funding
Open access funded by Max Planck Society.
Notes
The authors declare no competing financial interest.

Biographies

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Alexander Fellows obtained his MA and MSci in Natural Sciences from the University of Cambridge, United Kingdom, after which he stayed on to complete a PhD in Chemistry. In 2022, he commenced postdoctoral research at the Fritz Haber Institute in Berlin. His research interests surround the connection between molecular structure and macroscopic behavior at interfaces, with a particular focus on biophysical systems.

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Álvaro Díaz Duque obtained his undergraduate degree in Chemistry at the National University of Colombia and his MSc. in Chemistry at the Free University of Berlin. In 2021, he started his PhD at the Fritz Haber Institute in Berlin, in the field of second-order nonlinear vibrational spectroscopy of aqueous and charged aqueous interfaces. He is interested in understanding the dynamics and thermodynamics of processes governing electrified interfaces.

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Louis Lehmann obtained a B.Sc. in Chemistry from the Technische Universität Berlin and a M.Sc. in Computational Sciences from the Freie Universität Berlin in 2021. He is currently pursuing a Ph.D. in Physics at the Freie Universität Berlin. His research focuses on the theoretical prediction of nonlinear spectroscopy in aqueous systems.

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Roland Netz studied physics at the Technical University of Berlin and at MIT and received his Ph.D. in 1994 from the University of Cologne. After several postdoctoral positions he was appointed associate professor of physics at the LMU Munich in 2002 and full professor of physics at the TU Munich in 2004. Since 2011 he has held a chair in theoretical biosoft-matter physics at the Freie Universitä Berlin. His research focuses on the structure, dynamics and spectroscopy of water, proteins, polymers and charged systems using quantum, atomistic, and coarse-grained simulations as well as theoretical approaches.

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Martin Wolf received his PhD at Freie Universität Berlin, Germany, with Gerhard Ertl. After a postdoc in Austin, Texas, he set up a femtosecond surface spectroscopy laboratory at the Fritz Haber Institute of the MPG and was also visiting scientist at IBM Yorktown Heights. In 2000 he was appointed as full professor in experimental physics at Freie Universität Berlin and since 2008 he is director of the Department of Physical Chemistry of the Fritz Haber Institute. His research focuses on spatial-temporal dynamics of elementary excitations at interfaces.

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Martin Thämer studied Chemistry at the University of Münster, Germany and obtained his PhD in Physical Chemistry from the Technical University of Munich in 2012. After Postdoc positions at the Massachusetts Institute for Technology and the University of Chicago, he joined the Fritz-Haber-Institute in Berlin where he started a research group in 2019 that focusses on the investigation of molecular structures at interfaces using nonlinear spectroscopy and microscopy methods.

Acknowledgments

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The authors would like to acknowledge the Deutsche Forschungsgemeinschaft (DFG) for funding (Project-ID 221545957 - SFB 1078/C1).

References

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A review on the characterization of the surface mol. structure of polymeric materials using sum frequency generation vibrational spectroscopy. The in situ detection of the surface structures of some common polymers in air, surface segregation of small end groups, polymer surface restructuring in water and step-wise changed polymer blend surfaces are discussed. Studies of surface glass transition and surface structures modified by rubbing, plasma deposition, UV irradn., O ion and radical irradn. and wet etching also are discussed.
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Eisenthal, K. B. Liquid Interfaces Probed by Second-Harmonic and Sum-Frequency Spectroscopy. Chem. Rev. 1996, 96 (4), 1343– 1360, DOI: 10.1021/cr9502211
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Liquid Interfaces Probed by Second-Harmonic and Sum-Frequency Spectroscopy
Eisenthal, K. B.
Chemical Reviews (Washington, D. C.) (1996), 96 (4), 1343-1360CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review with ∼ 99 refs. on SHG and sum-frequency spectroscopy studies of liq. interfaces. Mol. orientations, chem. reactions, structures, phase transitions, and vibrational spectra can be probed by these methods. Electrochem., environmental, and photochem. examples are described.
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Johnson, C. M.; Baldelli, S. Vibrational Sum Frequency Spectroscopy Studies of the Influence of Solutes and Phospholipids at Vapor/Water Interfaces Relevant to Biological and Environmental Systems. Chem. Rev. 2014, 114 (17), 8416– 8446, DOI: 10.1021/cr4004902
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Vibrational Sum Frequency Spectroscopy Studies of the Influence of Solutes and Phospholipids at Vapor/Water Interfaces Relevant to Biological and Environmental Systems
Johnson, C. Magnus; Baldelli, Steven
Chemical Reviews (Washington, DC, United States) (2014), 114 (17), 8416-8446CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review. Vibrational sum frequency spectroscopy (VSFS) is a second-order nonlinear vibrational laser spectroscopy technique, with the unique property that it, under the elec. dipole approxn., is surface and interface specific. By their amphiphilic nature, surfactants have a detg. effect on water properties at the interface. The water surface is also influenced by sol. and less surface active mols. Solutes have more or less activity at the air/liq. interface and in these systems the water structure and the solute structure are mutually influenced. The weaker of the interactions between the solvent and solute is exemplified in the gas phase mols./water systems. Dissolved gases are also able to affect the water surface as the interface is a barrier they must pass in order to become solvated. At equil. the mols. are present in both the gas and soln. phase; however, the surface state might be stable enough to have a significant occupation. In this case the water surface structure has a large impact on the surface complexes. Examples of the effect of phospholipids and solutes, such as aldehydes, organosulfur compds., alcs., nitriles, acids, salts, and gases, are discussed.
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Shultz, M. J.; Schnitzer, C.; Simonelli, D.; Baldelli, S. Sum Frequency Generation Spectroscopy of the Aqueous Interface: Ionic and Soluble Molecular Solutions. Int. Rev. Phys. Chem. 2000, 19 (1), 123– 153, DOI: 10.1080/014423500229882
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Sum frequency generation spectroscopy of the aqueous interface. Ionic and soluble molecular solutions
Shultz, Mary Jane; Schnitzer, Cheryl; Simonelli, Danielle; Baldelli, Steve
International Reviews in Physical Chemistry (2000), 19 (1), 123-153CODEN: IRPCDL; ISSN:0144-235X. (Taylor & Francis Ltd.)
A review with 115 refs. The liq. interface of aq. solns. is of central importance to numerous phenomena from cloud processing of combustion generated oxides to corrosion degrdn. of structural materials to transport across cell membranes. Despite the importance of this interface, little mol.-level information was known about it prior to the last decade-and-a-half. Mol.-level information is important not only for a fundamental understanding of processes at interfaces, but also for predicting methods for diminishing deleterious effects. Recently, the non-linear spectroscopic method, sum frequency generation (SFG), was applied to the investigation of the structure of the liq. interface. This review focuses on the liq.-air interface of aq. solns. contg. sol., ionic species-H2SO4, HNO3, HCl, alkali sulfates and bisulfates, NaCl and NaNO3-as well as sol. mol. species-glycerol, H2SO4 and NH3. Ionic materials influence the structure of water at the interface through an elec. double layer which arises from the differential distribution of anions and cations near the interface. Due to the extreme size of the proton, the strongest field is generated by acidic materials. As the concn. of these ionic materials increases, ion pairs form diminishing the strength of the double layer. This enables the ion-pair complex to penetrate to the interface and either displace water or bind it into hydrated complexes. Sol. materials of lower surface tension partition to the interface and either displace water from the interface or bind water into hydrated complexes. In particular, the conjectured NH3-water complex on aq. solns. is obsd. and it is detd. to tilt 34-38° from the normal.
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Chen, Z. Investigating Buried Polymer Interfaces Using Sum Frequency Generation Vibrational Spectroscopy. Prog. Polym. Sci. 2010, 35 (11), 1376– 1402, DOI: 10.1016/j.progpolymsci.2010.07.003
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Investigating buried polymer interfaces using sum frequency generation vibrational spectroscopy
Chen, Zhan
Progress in Polymer Science (2010), 35 (11), 1376-1402CODEN: PRPSB8; ISSN:0079-6700. (Elsevier Ltd.)
A review. This paper reviews recent progress in the studies of buried polymer interfaces using sum frequency generation (SFG) vibrational spectroscopy. Both buried solid/liq. and solid/solid interfaces involving polymeric materials are discussed. SFG studies of polymer/water interfaces show that different polymers exhibit varied surface restructuring behavior in water, indicating the importance of probing polymer/water interfaces in situ. SFG has also been applied to the investigation of interfaces between polymers and other liqs. It has been found that mol. interactions at such polymer/liq. interfaces dictate interfacial polymer structures. The mol. structures of silane mols., which are widely used as adhesion promoters, have been investigated using SFG at buried polymer/silane and polymer/polymer interfaces, providing mol.-level understanding of polymer adhesion promotion. The mol. structures of polymer/solid interfaces have been examd. using SFG with several different exptl. geometries. These results have provided mol.-level information about polymer friction, adhesion, interfacial chem. reactions, interfacial electronic properties, and the structure of layer-by-layer deposited polymers. Such research has demonstrated that SFG is a powerful tool to probe buried interfaces involving polymeric materials, which are difficult to study by conventional surface sensitive anal. techniques.
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Yan, E. C. Y.; Fu, L.; Wang, Z.; Liu, W. Biological Macromolecules at Interfaces Probed by Chiral Vibrational Sum Frequency Generation Spectroscopy. Chem. Rev. 2014, 114 (17), 8471– 8498, DOI: 10.1021/cr4006044
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Biological Macromolecules at Interfaces Probed by Chiral Vibrational Sum Frequency Generation Spectroscopy
Yan, Elsa C. Y.; Fu, Li; Wang, Zhuguang; Liu, Wei
Chemical Reviews (Washington, DC, United States) (2014), 114 (17), 8471-8498CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review of on the exptl. and theor. developments of chiral vibrational SFG spectroscopy and its applications in probing biomacromols. at interfaces during the past decade.
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Hosseinpour, S.; Roeters, S. J.; Bonn, M.; Peukert, W.; Woutersen, S.; Weidner, T. Structure and Dynamics of Interfacial Peptides and Proteins from Vibrational Sum-Frequency Generation Spectroscopy. Chem. Rev. 2020, 120 (7), 3420– 3465, DOI: 10.1021/acs.chemrev.9b00410
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Structure and Dynamics of Interfacial Peptides and Proteins from Vibrational Sum-Frequency Generation Spectroscopy
Hosseinpour, Saman; Roeters, Steven J.; Bonn, Mischa; Peukert, Wolfgang; Woutersen, Sander; Weidner, Tobias
Chemical Reviews (Washington, DC, United States) (2020), 120 (7), 3420-3465CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review. Proteins at interfaces play an important role in cell biol., immunol., bioengineering, and biomimetic material design. Many biol. processes are based on interfacial protein action, ranging from cellular communication to immune responses and the protein-driven mineralization of bone. Despite the importance of interfacial proteins, comparatively little is known about their structure. The std. methods for studying cryst. or soln.-phase proteins (X-ray diffraction and NMR) are not well suited for studying proteins at interfaces, and for these proteins, we still lack a corresponding technique that can provide the same level of structural resolns. This is not surprising in view of the challenges involved in probing the structure of proteins within monomol. films assembled at a very thin interface in situ. Vibrational sum-frequency generation (SFG) spectroscopy has the potential to overcome this "resoln. gap" and investigate the structure and dynamics of proteins at interfaces at the mol. level and with subpicosecond time resoln. While SFG studies were initially limited to simple model peptides, the past decade has seen a dramatic advancement of exptl. techniques and data anal. methods, which has made it possible to also study interfacial proteins and their folding, binding, orientation, hydration, and dynamics. In this review, we first explain the principles of protein SFG and the exptl. and theor. methods to measure and analyze protein SFG spectra. Then we give an extensive overview of the interfacial proteins studied thus far with SFG. We highlight representative examples to demonstrate recent advances in probing the structure of proteins at the interfaces of liqs., membranes, minerals, and synthetic materials.
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Lambert, A. G.; Davies, P. B.; Neivandt, D. J. Implementing the Theory of Sum Frequency Generation Vibrational Spectroscopy: A Tutorial Review. Appl. Spectrosc. Rev. 2005, 40 (2), 103– 145, DOI: 10.1081/ASR-200038326
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Implementing the theory of sum frequency generation vibrational spectroscopy: a tutorial review
Lambert, Alex G.; Davies, Paul B.; Neivandt, David J.
Applied Spectroscopy Reviews (2005), 40 (2), 103-145CODEN: APSRBB; ISSN:0570-4928. (Taylor & Francis, Inc.)
A review. The interfacial regions between bulk media, although often comprising only a fraction of the material present, are frequently the site of reactions and phenomena that dominate the macroscopic properties of the entire system. Spectroscopic investigations of such interfaces are often hampered by the lack of surface specificity of most available techniques. Sum frequency generation vibrational spectroscopy (SFS) is a non-linear optical technique which provides vibrational spectra of mols. solely at interfaces. The spectra may be analyzed to provide the polar orientation, mol. conformation, and av. tilt angle of the adsorbate to the surface normal. This article is aimed at newcomers to the field of SFS, and via a tutorial approach will present and develop the general sum frequency equations and then demonstrate how the fundamental theory elucidates the important exptl. properties of SFS.
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Shen, Y. R. Revisiting the Basic Theory of Sum-Frequency Generation. J. Chem. Phys. 2020, 153 (18), 180901 DOI: 10.1063/5.0030947
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Revisiting the basic theory of sum-frequency generation
Shen, Y. R.
Journal of Chemical Physics (2020), 153 (18), 180901CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
The basic theory of sum-frequency generation (SFG) is revisited. A rigorous derivation showing that linear optical transmission and reflection at an interface result from the interference of the incident wave and induced radiation wave in a medium is presented. The derivation is extended to SFG in a medium with a finite interface layer to see how SFG evolves. Detailed description on interface vs bulk and elec. dipole (ED) vs elec. quadrupole (EQ) contribution to SFG are provided with essentially no model dependence, putting the theory of SFG on a solid ground and removing possible existing confusions. Elec.-quadrupole contributions to SFG from the interface and bulk are discussed. It is seen that there is a relevant bulk EQ contribution intrinsically inseparable in measurement from the interface ED contribution but plays a major role among all EQ contributions; its importance relative to the ED part can only be judged by referring to the established ref. cases. (c) 2020 American Institute of Physics.
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Patterson, J. E. The Nonresonant Sum-Frequency Generation Response: The Not-so-Silent Partner. J. Chem. Phys. 2024, 161 (6), 60901, DOI: 10.1063/5.0221401
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Zhang, Y.; de Aguiar, H. B.; Hynes, J. T.; Laage, D. Water Structure, Dynamics, and Sum-Frequency Generation Spectra at Electrified Graphene Interfaces. J. Phys. Chem. Lett. 2020, 11 (3), 624– 631, DOI: 10.1021/acs.jpclett.9b02924
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Water Structure, Dynamics, and Sum-Frequency Generation Spectra at Electrified Graphene Interfaces
Zhang, Yiwei; de Aguiar, Hilton B.; Hynes, James T.; Laage, Damien
Journal of Physical Chemistry Letters (2020), 11 (3), 624-631CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
The properties of water at an electrified graphene electrode are studied via classical mol. dynamics simulations with a const. potential approach. We show that the value of the applied electrode potential has dramatic effects on the structure and dynamics of interfacial water mols. While a pos. potential slows down the reorientational and translational dynamics of water, an increasing neg. potential first accelerates the interfacial water dynamics before a deceleration at very large magnitude potential values. Further, our spectroscopic calcns. indicate that the water rearrangements induced by electrified interfaces can be probed exptl. In particular, the calcd. water vibrational sum-frequency generation (SFG) spectra show that SFG specifically reports on the first two water layers at 0 V but that at larger magnitude applied potentials the resulting static field induces long-range contributions to the spectrum. Electrified graphene interfaces provide promising paradigm systems for comprehending both short- and long-range neighboring aq. system impacts.
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Fellows, A. P.; Duque, Á. D.; Balos, V.; Lehmann, L.; Netz, R. R.; Wolf, M.; Thämer, M. How Thick Is the Air–Water Interface?─A Direct Experimental Measurement of the Decay Length of the Interfacial Structural Anisotropy. Langmuir 2024, 40 (35), 18760– 18772, DOI: 10.1021/acs.langmuir.4c02571
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Morita, A.; Hynes, J. T. A Theoretical Analysis of the Sum Frequency Generation Spectrum of the Water Surface. Chem. Phys. 2000, 258 (2), 371– 390, DOI: 10.1016/S0301-0104(00)00127-0
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A theoretical analysis of the sum frequency generation spectrum of the water surface
Morita, A.; Hynes, J. T.
Chemical Physics (2000), 258 (2-3), 371-390CODEN: CMPHC2; ISSN:0301-0104. (Elsevier Science B.V.)
The present paper provides a theor. anal. of the sum frequency generation (SFG) spectrum of the water surface in the OH stretch mode frequency region based on ab initio MO theory and mol. dynamics simulation homol. dynamics simulation. The environmental effects on the normal modes of OH stretching vibration, their frequency shifts and hyperpolarizability are formulated and tested. The simulated SFG spectrum reproduces exptl. results quite well. The surface susceptibilities of the dangling bond and H-bonded bands have opposite signs in their imaginary parts, which indicates opposite OH directions at the surface assocd. with these bands. The former band turns out to be sensitive only to the top monolayer, and the latter band to a few top monolayers. Further anal. reveals that those 2 bands arise from quite different types of mol. orientations. The assignment of the SFG spectrum is also analyzed in terms of the sym./antisym. character of the OH stretching modes and the degree of mode delocalization.
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Ji, N.; Ostroverkhov, V.; Tian, C. S.; Shen, Y. R. Characterization of Vibrational Resonances of Water-Vapor Interfaces by Phase-Sensitive Sum-Frequency Spectroscopy. Phys. Rev. Lett. 2008, 100 (9), 96102, DOI: 10.1103/PhysRevLett.100.096102
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Nihonyanagi, S.; Ishiyama, T.; Lee, T.; Yamaguchi, S.; Bonn, M.; Morita, A.; Tahara, T. Unified Molecular View of the Air/Water Interface Based on Experimental and Theoretical χ(2) Spectra of an Isotopically Diluted Water Surface. J. Am. Chem. Soc. 2011, 133 (42), 16875– 16880, DOI: 10.1021/ja2053754
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Unified Molecular View of the Air/Water Interface Based on Experimental and Theoretical χ(2) Spectra of an Isotopically Diluted Water Surface
Nihonyanagi, Satoshi; Ishiyama, Tatsuya; Lee, Touk-kwan; Yamaguchi, Shoichi; Bonn, Mischa; Morita, Akihiro; Tahara, Tahei
Journal of the American Chemical Society (2011), 133 (42), 16875-16880CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The energetically unfavorable termination of the hydrogen-bonded network of water mols. at the air/water interface causes mol. rearrangement to minimize the free energy. The long-standing question is how water minimizes the surface free energy. The combination of advanced, surface-specific nonlinear spectroscopy and theor. simulation provides new insights. The complex χ(2) spectra of isotopically dild. water surfaces obtained by heterodyne-detected sum frequency generation spectroscopy and mol. dynamics simulation show excellent agreement, assuring the validity of the microscopic picture given in the simulation. The present study indicates that there is no ice-like structure at the surface - in other words, there is no increase of tetrahedrally coordinated structure compared to the bulk - but that there are water pairs interacting with a strong hydrogen bond at the outermost surface. Intuitively, this can be considered a consequence of the lack of a hydrogen bond toward the upper gas phase, enhancing the lateral interaction at the boundary. This study also confirms that the major source of the isotope effect on the water χ(2) spectra is the intramol. anharmonic coupling, i.e., Fermi resonance.
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Inoue, K.; Ahmed, M.; Nihonyanagi, S.; Tahara, T. Reorientation-Induced Relaxation of Free OH at the Air/Water Interface Revealed by Ultrafast Heterodyne-Detected Nonlinear Spectroscopy. Nat. Commun. 2020, 11 (1), 5344, DOI: 10.1038/s41467-020-19143-8
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Reorientation-induced relaxation of free OH at the air/water interface revealed by ultrafast heterodyne-detected nonlinear spectroscopy
Inoue, Ken-ichi; Ahmed, Mohammed; Nihonyanagi, Satoshi; Tahara, Tahei
Nature Communications (2020), 11 (1), 5344CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
The uniqueness of water originates from its three-dimensional hydrogen-bond network, but this hydrogen-bond network is suddenly truncated at the interface and non-hydrogen-bonded OH (free OH) appears. Although this free OH is the most characteristic feature of interfacial water, the mol.-level understanding of its dynamic property is still limited due to the tech. difficulty. We study ultrafast vibrational relaxation dynamics of the free OH at the air/water interface using time-resolved heterodyne-detected vibrational sum frequency generation (TR-HD-VSFG) spectroscopy. With the use of singular value decompn. (SVD) anal., the vibrational relaxation (T1) times of the free OH at the neat H2O and isotopically-dild. water interfaces are detd. to be 0.87 ± 0.06 ps (neat H2O), 0.84 ± 0.09 ps (H2O/HOD/D2O = 1/2/1), and 0.88 ± 0.16 ps (H2O/HOD/D2O = 1/8/16). The absence of the isotope effect on the T1 time indicates that the main mechanism of the vibrational relaxation of the free OH is reorientation of the topmost water mols. The detd. sub-picosecond T1 time also suggests that the free OH reorients diffusively without the switching of the hydrogen-bond partner by the topmost water mol.
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Hsiao, Y.; Chou, T.-H.; Patra, A.; Wen, Y.-C. Momentum-Dependent Sum-Frequency Vibrational Spectroscopy of Bonded Interface Layer at Charged Water Interfaces. Sci. Adv. 2023, 9 (15), eadg2823 DOI: 10.1126/sciadv.adg2823
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Chiang, K.-Y.; Seki, T.; Yu, C. C.; Ohto, T.; Hunger, J.; Bonn, M.; Nagata, Y. The Dielectric Function Profile Across the Water Interface Through Surface-Specific Vibrational Spectroscopy and Simulations. Proc. Natl. Acad. Sci. U. S. A. 2022, 119 (36), e2204156119 DOI: 10.1073/pnas.2204156119
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Stiopkin, I. V.; Weeraman, C.; Pieniazek, P. A.; Shalhout, F. Y.; Skinner, J. L.; Benderskii, A. V. Hydrogen Bonding at the Water Surface Revealed by Isotopic Dilution Spectroscopy. Nature 2011, 474 (7350), 192– 195, DOI: 10.1038/nature10173
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Hydrogen bonding at the water surface revealed by isotopic dilution spectroscopy
Stiopkin, Igor V.; Weeraman, Champika; Pieniazek, Piotr A.; Shalhout, Fadel Y.; Skinner, James L.; Benderskii, Alexander V.
Nature (London, United Kingdom) (2011), 474 (7350), 192-195CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
The air-water interface is perhaps the most common liq. interface. It covers more than 70% of the Earth surface and strongly affects atm., aerosol and environmental chem. The air-water interface has also attracted much interest as a model system that allows rigorous tests of theory, with one fundamental question being just how thin it is. Theor. studies have suggested a surprisingly short healing length of about 3 Å (1 Å = 0.1 nm), with the bulk-phase properties of water recovered within the top few monolayers. However, direct exptl. evidence was elusive owing to the difficulty of depth-profiling the liq. surface on the Å scale. Most phys., chem., and biol. properties of water, such as viscosity, solvation, wetting and the hydrophobic effect, are detd. by its H-bond network. This can be probed by observing the lineshape of the OH-stretch mode, the frequency shift of which is related to the H-bond strength. Here we report a combined exptl. and theor. study of the air-water interface using surface-selective heterodyne-detected vibrational sum frequency spectroscopy to focus on the free OD transition found only in the topmost water layer. By using deuterated water and isotopic diln. to reveal the vibrational coupling mechanism, we find that the free OD stretch is affected only by intramol. coupling to the stretching of the other OD group on the same mol. The other OD stretch frequency indicates the strength of one of the first H bonds encountered at the surface; this is the donor H bond of the water mol. straddling the interface, which we find to be only slightly weaker than bulk-phase water H bonds. We infer from this observation a remarkably fast onset of bulk-phase behavior on crossing from the air into the water phase.
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Pieniazek, P. A.; Tainter, C. J.; Skinner, J. L. Interpretation of the Water Surface Vibrational Sum-Frequency Spectrum. J. Chem. Phys. 2011, 135 (4), 44701, DOI: 10.1063/1.3613623
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Pezzotti, S.; Galimberti, D. R.; Gaigeot, M.-P. 2D H-Bond Network as the Topmost Skin to the Air–Water Interface. J. Phys. Chem. Lett. 2017, 8 (13), 3133– 3141, DOI: 10.1021/acs.jpclett.7b01257
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2D H-Bond Network as the Topmost Skin to the Air-Water Interface
Pezzotti, Simone; Galimberti, Daria Ruth; Gaigeot, Marie-Pierre
Journal of Physical Chemistry Letters (2017), 8 (13), 3133-3141CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
A description of the structure of water at the interface with the air (liq.-vapor LV interface) from state-of-the-art DFT-based mol. dynamics simulations is provided. A two-dimensional (2D) H-bond extended network has been identified and fully characterized, demonstrating that interfacial water is organized into a 2D sheet with H-bonds oriented parallel to the instantaneous surface and following its spatial and temporal oscillations. By analyzing the nonlinear vSFG (vibrational sum frequency generation) spectrum of the LV interface in terms of layer-by-layer signal, it is demonstrate that the 2D water sheet is solely responsible for the spectral signatures, hence providing the interfacial 3.5 Å thickness effectively probed in nonlinear interfacial spectroscopy. The 2D H-bond network unraveled here is the essential key to rationalize macroscopic properties of water-air interfaces, as demonstrated here for spectroscopy and the surface potential.
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Mukamel, S. Principles of Nonlinear Optical Spectroscopy; Oxford University Press: Oxford, 1995.
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Wang, H. F.; Gan, W.; Lu, R.; Rao, Y.; Wu, B. H. Quantitative Spectral and Orientational Analysis in Surface Sum Frequency Generation Vibrational Spectroscopy (SFG-VS). Int. Rev. Phys. Chem. 2005, 24 (2), 191– 256, DOI: 10.1080/01442350500225894
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Quantitative spectral and orientational analysis in surface sum frequency generation vibrational spectroscopy (SFG-VS)
Wang, Hong-Fei; Gan, Wei; Lu, Rong; Rao, Yi; Wu, Bao-Hua
International Reviews in Physical Chemistry (2005), 24 (2), 191-256CODEN: IRPCDL; ISSN:0144-235X. (Taylor & Francis Ltd.)
A review. Sum frequency generation vibrational spectroscopy (SFG-VS) was proven to be a uniquely effective spectroscopic technique in the study of mol. structure and conformations, as well as the dynamics of mol. interfaces. The ability to apply SFG-VS to complex mol. interfaces was limited by the ability to abstr. quant. information from SFG-VS expts. The authors try to make assessments of the limitations, issues and techniques as well as methodols. in quant. orientational and spectral anal. with SFG-VS. Based on these assessments, the authors also try to summarize recent developments in methodologies on quant. orientational and spectral anal. in SFG-VS, and their applications to detailed anal. of SFG-VS data of various vapor/neat liq. interfaces. A rigorous formulation of the polarization null angle (PNA) method is given for accurate detn. of the orientational parameter D = 〈cos θ 〉/〈cos3 θ〉, and comparison between the PNA method with the commonly used polarization intensity ratio (PIR) method is discussed. The polarization and incident angle dependencies of the SFG-VS intensity are reviewed, in the light of how exptl. arrangements can be optimized to effectively abstr. crucial information from the SFG-VS expts. The values and models of the local field factors in the mol. layers are discussed. To examine the validity and limitations of the bond polarizability deriv. model, the general expressions for mol. hyperpolarizability tensors and their expression with the bond polarizability deriv. model for C3v, C2v and C∞v mol. groups are given in the 2 appendixes. The bond polarizability deriv. model can quant. describe many aspects of the intensities obsd. in the SFG-VS spectrum of the vapor/neat liq. interfaces in different polarizations. Using the polarization anal. in SFG-VS, polarization selection rules or guidelines are developed for assignment of the SFG-VS spectrum. Using the selection rules, SFG-VS spectra of vapor/diol, and vapor/n-normal alc. (n ∼ 1-8) interfaces are assigned, and some of the ambiguity and confusion, as well as their implications in previous IR and Raman assignment, are duly discussed. The ability to assign a SFG-VS spectrum using the polarization selection rules makes SFG-VS not only an effective and useful vibrational spectroscopy technique for interface studies, but also a complementary vibrational spectroscopy method in general condensed phase studies. These developments will put quant. orientational and spectral anal. in SFG-VS on a more solid foundation. The formulations, concepts and issues discussed in this review are expected to find broad applications for studies on mol. interfaces in the future.
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Balos, V.; Garling, T.; Duque, A. D.; John, B.; Wolf, M.; Thämer, M. Phase-Sensitive Vibrational Sum and Difference Frequency-Generation Spectroscopy Enabling Nanometer-Depth Profiling at Interfaces. J. Phys. Chem. C 2022, 126 (26), 10818– 10832, DOI: 10.1021/acs.jpcc.2c01324
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Fellows, A. P.; Balos, V.; John, B.; Díaz Duque, Á.; Wolf, M.; Thämer, M. Obtaining Extended Insight into Molecular Systems by Probing Multiple Pathways in Second-Order Nonlinear Spectroscopy. J. Chem. Phys. 2023, 159 (16), 164201 DOI: 10.1063/5.0169534
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Gonella, G.; Lütgebaucks, C.; de Beer, A. G. F.; Roke, S. Second Harmonic and Sum-Frequency Generation from Aqueous Interfaces Is Modulated by Interference. J. Phys. Chem. C 2016, 120 (17), 9165– 9173, DOI: 10.1021/acs.jpcc.5b12453
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Second Harmonic and Sum-Frequency Generation from Aqueous Interfaces Is Modulated by Interference
Gonella, Grazia; Luetgebaucks, Cornelis; de Beer, Alex G. F.; Roke, Sylvie
Journal of Physical Chemistry C (2016), 120 (17), 9165-9173CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
The interfacial region of aq. systems also known as the elec. double layer can be characterized on the mol. level with 2nd harmonic and sum-frequency generation (SHG/SFG). SHG and SFG are surface specific methods for isotropic liqs. Here, the authors model the SHG/SFG intensity in reflection, transmission, and scattering geometry taking into account the spatial variation of all fields. In the presence of a surface electrostatic field, interference effects, which originate from oriented H2O mols. on a length scale over which the potential decays, can strongly modify the probing depth as well as the expected intensity at ionic strengths <10-3 M. For reflection expts. this interference phenomenon leads to a significant redn. of the SHG/SFG intensity. Transmission mode expts. from aq. interfaces are hardly influenced. For SHG/SFG scattering expts. the same interference increases intensity and to modified scattering patterns. The predicted scattering patterns are verified exptl.
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Hu, X.-H.; Wei, F.; Wang, H.; Wang, H.-F. α-Quartz Crystal as Absolute Intensity and Phase Standard in Sum-Frequency Generation Vibrational Spectroscopy. J. Phys. Chem. C 2019, 123 (24), 15071– 15086, DOI: 10.1021/acs.jpcc.9b03202
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α-Quartz Crystal as Absolute Intensity and Phase Standard in Sum-Frequency Generation Vibrational Spectroscopy
Hu, Xiao-Hua; Wei, Feng; Wang, Hui; Wang, Hong-Fei
Journal of Physical Chemistry C (2019), 123 (24), 15071-15086CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Quartz crystal was used as intensity and phase ref. std. in sum-frequency generation (SFG) and other nonlinear spectroscopy measurements. The authors examd. the issues regarding the most widely used z-cut α-quartz in SFG intensity and phase measurements, through systematic comparison of optical rotation, piezoelec., and SFG vibrational spectroscopy (SFG-VS) measurement results. For quartz crystal used as SFG-VS intensity std., the authors examd. the issue on the spectroscopic flatness of quartz crystal and Au film in the broad range of 1000-4000 cm-1, and at 2000-4000 cm-1, both quartz crystal and Au film are spectrally flat and are suitable for intensity ref. in the SFG-VS measurement. For quartz crystal being used as an abs. phase std., the detn. of the abs. direction of the quartz crystal is crucial, and in the past this was defined and detd. using piezoelec. measurement. The 1949 and 1945 IRE (Institute of Radio Engineers) stds. of defining the coordinates system based on the piezoelec. measurement for the quartz unit cell and crystal are often inconvenient in the applications to the nonlinear spectroscopy measurement and may cause confusion. Therefore, the authors proposed a new SFG/SHG (2nd-harmonic generation) convention to consistently define the quartz crystal coordinates system. Then, the internal heterodyne phase-resolved (IHPR) SFG-VS measurement of adsorbed mol. layers, e.g., OTS (octadecyltrichlorosilane), on the quartz crystal surface can be used to self-consistently det. the abs. azimuthal orientation direction of the quartz crystal, independent from the use of the piezoelec. measurement. In the end, the authors also examd. the issue regarding the abs. phase of the hyperpolarizability of various mol. groups, as well as quartz, under proper coordinates system. These results are useful for the future applications to the nonlinear spectroscopy, particularly the nonlinear spectroscopy for surface and interface studies.
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Thämer, M.; Garling, T.; Campen, R. K.; Wolf, M. Quantitative Determination of the Nonlinear Bulk and Surface Response from Alpha-Quartz Using Phase Sensitive SFG Spectroscopy. J. Chem. Phys. 2019, 151 (6), 064707, DOI: 10.1063/1.5109868
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Baldelli, S.; Schnitzer, C.; Shultz, M. J.; Campbell, D. J. Sum Frequency Generation Investigation of Water at the Surface of H2O/H2SO4 Binary Systems. J. Phys. Chem. B 1997, 101 (49), 10435– 10441, DOI: 10.1021/jp972376d
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Sum Frequency Generation Investigation of Water at the Surface of H2O/H2SO4 Binary Systems
Baldelli, Steve; Schnitzer, Cheryl; Shultz, Mary Jane; Campbell, D. J.
Journal of Physical Chemistry B (1997), 101 (49), 10435-10441CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
Structural data about water at the air/water interface of sulfuric acid solns. have been obtained with sum frequency generation spectroscopy. Sulfuric acid significantly affects the orientation of water at the interface. With as little as 0.01x (mole fraction) sulfuric acid, water is more highly oriented compared with the pure water surface. Surface water with one hydrogen free of hydrogen bonding, a "free OH", decreases in concn. as the sulfuric acid concn. is increased. Finally, there are no sulfuric acid free OH groups projecting out of the surface for solns. from 0.01x to 0.9x sulfuric acid. Observations of the structure of water are consistent with the fact that sulfuric acid/water solns. change from ionic in nature at low concns. (<0.01x) to ion pair complexes/hydrates at high concns. (>0.4x).
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Medders, G. R.; Paesani, F. Dissecting the Molecular Structure of the Air/Water Interface from Quantum Simulations of the Sum-Frequency Generation Spectrum. J. Am. Chem. Soc. 2016, 138 (11), 3912– 3919, DOI: 10.1021/jacs.6b00893
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Dissecting the Molecular Structure of the Air/Water Interface from Quantum Simulations of the Sum-Frequency Generation Spectrum
Medders, Gregory R.; Paesani, Francesco
Journal of the American Chemical Society (2016), 138 (11), 3912-3919CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The mol. characterization of the air/water interface is a key step in understanding fundamental multiphase phenomena ranging from heterogeneous chem. processes in the atm. to the hydration of biomols. The apparent simplicity of the air/water interface, however, masks an underlying complexity assocd. with the dynamic nature of the water hydrogen-bond network that has so far hindered an unambiguous characterization of its microscopic properties. Here, we demonstrate that the application of quantum many-body mol. dynamics, which enables spectroscopically accurate simulations of water from the gas to the condensed phase, leads to a definitive mol.-level picture of the interface region. For the first time, excellent agreement is obtained between the simulated vibrational sum-frequency generation spectrum and the most recent state-of-the-art measurements, without requiring any empirical frequency shift or ad hoc scaling of the spectral intensity. A dissection of the spectral features demonstrates that a rigorous representation of nuclear quantum effects as well as of many-body energy and electrostatic contributions is necessary for a quant. reprodn. of the exptl. data. The accuracy of the simulations presented here indicates that quantum many-body mol. dynamics can enable predictive studies of aq. interfaces, which by complementing analogous exptl. measurements will provide unique mol. insights into multiphase and heterogeneous processes of relevance in chem., biol., materials science, and environmental research.
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Wang, J.; Chen, X.; Clarke, M. L.; Chen, Z. Detection of Chiral Sum Frequency Generation Vibrational Spectra of Proteins and Peptides at Interfaces in Situ. Proc. Natl. Acad. Sci. U. S. A. 2005, 102 (14), 4978– 4983, DOI: 10.1073/pnas.0501206102
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Detection of chiral sum frequency generation vibrational spectra of proteins and peptides at interfaces in situ
Wang, Jie; Chen, Xiaoyun; Clarke, Matthew L.; Chen, Zhan
Proceedings of the National Academy of Sciences of the United States of America (2005), 102 (14), 4978-4983CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
In this work, we demonstrate the feasibility to collect off-electronic resonance chiral sum frequency generation (SFG) vibrational spectra from interfacial proteins and peptides at the solid/liq. interface in situ. It is difficult to directly detect a chiral SFG vibrational spectrum from interfacial fibrinogen mols. By adopting an interference enhancement method, such a chiral SFG vibrational spectrum can be deduced from interference spectra between the normal achiral spectrum and the chiral spectrum. We found that the chiral SFG vibrational spectrum of interfacial fibrinogen was mainly contributed by the β-sheet structure. For a β-sheet peptide tachyplesin I, which may be quite ordered at the solid/liq. interface, chiral SFG vibrational spectra can be collected directly. We believe that these chiral signals are mainly contributed by elec. dipole contributions, which can dominate the chiroptical responses of uniaxial systems. For the first time, to our knowledge, this work indicates that the off-electronic resonance SFG technique is sensitive enough to collect chiral SFG vibrational spectra of interfacial proteins and peptides, providing more structural information to elucidate interfacial protein and peptide structures.
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Johnson, C. M.; Tyrode, E. Study of the Adsorption of Sodium Dodecyl Sulfate (SDS) at the Air/Water Interface: Targeting the Sulfate Headgroup Using Vibrational Sum Frequency Spectroscopy. Phys. Chem. Chem. Phys. 2005, 7 (13), 2635– 2640, DOI: 10.1039/b505219j
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Study of the adsorption of sodium dodecyl sulfate (SDS) at the air/water interface: targeting the sulfate headgroup using vibrational sum frequency spectroscopy
Johnson, C. Magnus; Tyrode, Eric
Physical Chemistry Chemical Physics (2005), 7 (13), 2635-2640CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
The surface sensitive technique vibrational sum frequency spectroscopy (VSFS), was used to study the adsorption behavior of SDS to the liq./vapor interface of aq. solns., specifically targeting the sulfate headgroup stretches. In the spectral region extending from 980 to 1850 cm-1, only the vibrations due to the SO3 group were detectable. The fitted amplitudes for the sym. SO3 stretch obsd. at 1070 cm-1 for the polarization combinations ssp and ppp, were seen to follow the adsorption isotherm calcd. from surface tension measurements. The orientation of the sulfate headgroup in the concn. range spanning from 1.0 mM to above the crit. micellar concn. (c.m.c.) was obsd. to remain const. within exptl. error, with the pseudo-C3 axis close to the surface normal. Furthermore, the effect of increasing amts. of NaCl at SDS concns. above c.m.c. was also studied, showing an increase of ≈12% in the fitted amplitude for the sym. SO3 stretch when increasing the ionic strength from 0-300 mM NaCl. Interestingly, the orientation of the SDS headgroup was also obsd. to remain const. within this concn. range and identical to the case without NaCl.
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Bell, G. R.; Bain, C. D.; Ward, R. N. Sum-Frequency Vibrational Spectroscopy of Soluble Surfactants at the Air/Water Interface. J. Chem. Soc. Faraday Trans. 1996, 92 (4), 515– 523, DOI: 10.1039/ft9969200515
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Sum-frequency vibrational spectroscopy of soluble surfactants at the air/water interface
Bell, Graham R.; Bain, Colin D.; Ward, Robert N.
Journal of the Chemical Society, Faraday Transactions (1996), 92 (4), 515-23CODEN: JCFTEV; ISSN:0956-5000. (Royal Society of Chemistry)
Vibrational spectra of seven surfactants adsorbed at the surface of aq. solns. have been obtained by IR-VIS sum-frequency generation. From these spectra, the degree of conformational disorder and the angle of the terminal Me group are inferred. In general, the no. of gauche conformations increases as the area per chain increases. The angle of the Me group, which is an indicator of the tilt of the hydrocarbon chains, is not simply related to the area per chain. Comparison of surfactants with the same chain length and area per mol. shows that the structure of the chain region of the monolayer is sensitive to the nature of the head group and not just to the packing d. Quant. models to explain peak intensities in sum-frequency spectra of surfactants are discussed critically.
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Ma, G.; Allen, H. C. DPPC Langmuir Monolayer at the Air-Water Interface: Probing the Tail and Head Groups by Vibrational Sum Frequency Generation Spectroscopy. Langmuir 2006, 22 (12), 5341– 5349, DOI: 10.1021/la0535227
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DPPC Langmuir Monolayer at the Air-Water Interface: Probing the Tail and Head Groups by Vibrational Sum Frequency Generation Spectroscopy
Ma, Gang; Allen, Heather C.
Langmuir (2006), 22 (12), 5341-5349CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
Dipalmitoylphosphatidylcholine (DPPC) is the predominant lipid component in lung surfactant. In this study, the Langmuir monolayer of deuterated dipalmitoylphosphatidylcholine (DPPC-d62) in the liq.-expanded (LE) phase and the liq.-condensed (LC) phase has been investigated at the air-water interface with broad bandwidth sum frequency generation (BBSFG) spectroscopy combined with a Langmuir film balance. Four moieties of the DPPC mol. are probed by BBSFG: the terminal Me (CD3) groups of the tails, the methylene (CD2) groups of the tails, the choline methyls (CH3) in the headgroup, and the phosphate in the headgroup. BBSFG spectra of the four DPPC moieties provide information about chain conformation, chain orientation, headgroup orientation, and headgroup hydration. These results provide a comprehensive picture of the DPPC phase behavior at the air-water interface. In the LE phase, the DPPC hydrocarbon chains are conformationally disordered with a significant no. of gauche configurations. In the LC phase, the hydrocarbon chains are in an all-trans conformation and are tilted from the surface normal by 25°. In addn., the orientations of the tail terminal Me groups are found to remain nearly unchanged with the variation of surface area. Qual. anal. of the BBSFG spectra of the choline Me groups suggests that these Me groups are tilted but lie somewhat parallel to the surface plane in both the LE and LC phases. The dehydration of the phosphate headgroup due to the LE-LC phase transition is obsd. through the frequency blue shift of the phosphate sym. stretch in the fingerprint region. In addn., implications for lung surfactant function from this work are discussed.
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Wang, Y.; Seki, T.; Yu, X.; Yu, C.-C.; Chiang, K.-Y.; Domke, K. F.; Hunger, J.; Chen, Y.; Nagata, Y.; Bonn, M. Chemistry Governs Water Organization at a Graphene Electrode. Nature 2023, 615 (7950), E1– E2, DOI: 10.1038/s41586-022-05669-y
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Gonella, G.; Backus, E. H. G.; Nagata, Y.; Bonthuis, D. J.; Loche, P.; Schlaich, A.; Netz, R. R.; Kühnle, A.; McCrum, I. T.; Koper Water at Charged Interfaces. Nat. Rev. Chem. 2021, 5 (7), 466– 485, DOI: 10.1038/s41570-021-00293-2
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Water at charged interfaces
Gonella, Grazia; Backus, Ellen H. G.; Nagata, Yuki; Bonthuis, Douwe J.; Loche, Philip; Schlaich, Alexander; Netz, Roland R.; Kuehnle, Angelika; McCrum, Ian T.; Koper, Marc T. M.; Wolf, Martin; Winter, Bernd; Meijer, Gerard; Campen, R. Kramer; Bonn, Mischa
Nature Reviews Chemistry (2021), 5 (7), 466-485CODEN: NRCAF7; ISSN:2397-3358. (Nature Portfolio)
A review. The ubiquity of aq. solns. in contact with charged surfaces and the realization that the mol.-level details of water-surface interactions often det. interfacial functions and properties relevant in many natural processes have led to intensive research. Even so, many open questions remain regarding the mol. picture of the interfacial organization and preferential alignment of water mols., as well as the structure of water mols. and ion distributions at different charged interfaces. While water, solutes and charge are present in each of these systems, the substrate can range from living tissues to metals. This diversity in substrates has led to different communities considering each of these types of aq. interface. In this Review, by considering water in contact with metals, oxides and biomembranes, we show the essential similarity of these disparate systems. While in each case the classical mean-field theories can explain many macroscopic and mesoscopic observations, it soon becomes apparent that such theories fail to explain phenomena for which mol. properties are relevant, such as interfacial chem. conversion. We highlight the current knowledge and limitations in our understanding and end with a view towards future opportunities in the field.
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Geiger, F. M. Second Harmonic Generation, Sum Frequency Generation, and χ (3): Dissecting Environmental Interfaces with a Nonlinear Optical Swiss Army Knife. Annu. Rev. Phys. Chem. 2009, 60 (1), 61– 83, DOI: 10.1146/annurev.physchem.59.032607.093651
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Second harmonic generation, sum frequency generation, and χ(3): dissecting environmental interfaces with a nonlinear optical Swiss army knife
Geiger, Franz M.
Annual Review of Physical Chemistry (2009), 60 (), 61-83CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews Inc.)
This review discusses recent advances in the nonlinear optics of environmental interfaces. We discuss the quant. aspects of the label-free approaches presented here and demonstrate that nonlinear optics has now assumed the role of a Swiss Army knife that can be used to dissect, with mol. detail, the fundamental and practical aspects of environmental interfaces and heterogeneous geochem. environments. In this work, nonlinear optical methods are applied to complex org. mols., such as veterinary antibiotics, and to small inorg. anions and cations, such as nitrate and chromate, or cadmium, zinc, and manganese. The environmental implications of the thermodn., kinetic, spectroscopic, structural, and electrochem. data are discussed.
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Yan, E. C. Y.; Liu, Y.; Eisenthal, K. B. New Method for Determination of Surface Potential of Microscopic Particles by Second Harmonic Generation. J. Phys. Chem. B 1998, 102 (33), 6331– 6336, DOI: 10.1021/jp981335u
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New Method for Determination of Surface Potential of Microscopic Particles by Second Harmonic Generation
Yan, Elsa C. Y.; Liu, Yan; Eisenthal, Kenneth B.
Journal of Physical Chemistry B (1998), 102 (33), 6331-6336CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
A new noninvasive method for detg. the surface electrostatic potential and surface charge d. of microscopic particles using 2nd harmonic generation (SHG) is described. The surface electrostatic properties of 1.05 μm polystyrene sulfate spheres in aq. soln. and that of 0.22 μm oil droplets in aq. emulsions are obtained. Comparisons of the surface potentials obtained from SHG with the zeta potential obtained from electrophoresis are in excellent agreement with theor. predictions.
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Wang, H.-F. Sum Frequency Generation Vibrational Spectroscopy (SFG-VS) for Complex Molecular Surfaces and Interfaces: Spectral Lineshape Measurement and Analysis plus Some Controversial Issues. Prog. Surf. Sci. 2016, 91 (4), 155– 182, DOI: 10.1016/j.progsurf.2016.10.001
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Sum frequency generation vibrational spectroscopy (SFG-VS) for complex molecular surfaces and interfaces: Spectral lineshape measurement and analysis plus some controversial issues
Wang, Hong-Fei
Progress in Surface Science (2016), 91 (4), 155-182CODEN: PSSFBP; ISSN:0079-6816. (Elsevier B.V.)
Sum-frequency generation vibrational spectroscopy (SFG-VS) was first developed in the 1980s and it has been proven a uniquely sensitive and surface/interface selective spectroscopic probe for characterization of the structure, conformation and dynamics of mol. surfaces and interfaces. In recent years, there have been many progresses in the development of methodol. and instrumentation in the SFG-VS toolbox that have significantly broadened the application to complex mol. surfaces and interfaces. In this review, after presenting a unified view on the theory and methodol. focusing on the SFG-VS spectral lineshape, as well as the new opportunities in SFG-VS applications with such developments, some of the controversial issues that have been puzzling the community are discussed. The aim of this review is to present to the researchers and students interested in mol. surfaces and interfacial sciences up-to-date perspectives complementary to the existing textbooks and reviews on SFG-VS.
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Backus, E. H. G.; Schaefer, J.; Bonn, M. Probing the Mineral–Water Interface with Nonlinear Optical Spectroscopy. Angew. Chemie Int. Ed. 2021, 60 (19), 10482– 10501, DOI: 10.1002/anie.202003085
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Covert, P. A.; Hore, D. K. Geochemical Insight from Nonlinear Optical Studies of Mineral–Water Interfaces. Annu. Rev. Phys. Chem. 2016, 67, 233– 257, DOI: 10.1146/annurev-physchem-040215-112300
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Geochemical Insight from Nonlinear Optical Studies of Mineral-Water Interfaces
Covert, Paul A.; Hore, Dennis K.
Annual Review of Physical Chemistry (2016), 67 (), 233-257CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews)
The physics and chem. of mineral-water interfaces are complex, even in idealized systems. Our need to understand this complexity is driven by both pure and applied sciences, i.e., by the need for basic understanding of earth systems and for the knowledge to mitigate our influences upon them. The second-order nonlinear optical techniques of second-harmonic generation and sum-frequency generation spectroscopy have proven adept at probing these types of interfaces. This review focuses on the contributions to geochem. made by nonlinear optical methods. The types of questions probed have included a basic description of the structure adopted by water mols. at the mineral interface, how flow and porosity affect this structure, adsorption of trace metal and org. species, and dissoln. mechanisms. We also discuss directions and challenges that lie ahead and the outlook for the continued use of nonlinear optical methods for studies of mineral-water boundaries.
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Wen, Y.-C.; Zha, S.; Liu, X.; Yang, S.; Guo, P.; Shi, G.; Fang, H.; Shen, Y. R.; Tian, C. Unveiling Microscopic Structures of Charged Water Interfaces by Surface-Specific Vibrational Spectroscopy. Phys. Rev. Lett. 2016, 116 (1), 016101, DOI: 10.1103/PhysRevLett.116.016101
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Unveiling microscopic structures of charged water interfaces by surface-specific vibrational spectroscopy
Wen, Yu-Chieh; Zha, Shuai; Liu, Xing; Yang, Shanshan; Guo, Pan; Shia, Guosheng; Fang, Haiping; Shen, Y. Ron; Tian, Chuanshan
Physical Review Letters (2016), 116 (1), 016101/1-016101/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
A sum-frequency spectroscopy scheme is developed that allows the measurement of vibrational spectra of the interfacial mol. structure of charged water interfaces. The application of this scheme to a prototype lipid-aq. interface as a demonstration reveals an interfacial hydrogen-bonding water layer structure that responds sensitively to the charge state of the lipid headgroup and its interaction with specific ions. This novel technique provides unique opportunities to search for better understanding of electrochem. and biol. aq. interfaces at a deeper mol. level.
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Zhao, X.; Ong, S.; Eisenthal, K. B. Polarization of Water Molecules at a Charged Interface. Second Harmonic Studies of Charged Monolayers at the Air/Water Interface. Chem. Phys. Lett. 1993, 202 (6), 513– 520, DOI: 10.1016/0009-2614(93)90041-X
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Polarization of water molecules at a charged interface. Second harmonic studies of charged monolayers at the air/water interface
Zhao, Xiaolin; Ong, Shaowei; Eisenthal, Kenneth B.
Chemical Physics Letters (1993), 202 (6), 513-20CODEN: CHPLBC; ISSN:0009-2614.
The second harmonic generation from charged monolayers at air/water interfaces is linearly related to the interface elec. potential. This dependence is due to the polarization of water mols. in the electrostatic field of the charged monolayer. The obsd. linear dependence of the second harmonic field on the interface elec. potential serves as the basis of a proposed method for obtaining the interface potential, which it should be noted is a nonintrusive optical method. The Gouy-Chapman model is valid at the air/water charged interface up to at least a 1M total electrolyte concn.
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Schaefer, J.; Gonella, G.; Bonn, M.; Backus, E. H. G. Surface-Specific Vibrational Spectroscopy of the Water/Silica Interface: Screening and Interference. Phys. Chem. Chem. Phys. 2017, 19 (25), 16875– 16880, DOI: 10.1039/C7CP02251D
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Surface-specific vibrational spectroscopy of the water/silica interface: screening and interference
Schaefer, Jan; Gonella, Grazia; Bonn, Mischa; Backus, Ellen H. G.
Physical Chemistry Chemical Physics (2017), 19 (25), 16875-16880CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
Surface-specific vibrational sum-frequency generation spectroscopy (V-SFG) is frequently used to obtain information about the mol. structure at charged interfaces. Here, we provide exptl. evidence that not only screening of surface charges but also interference limits the extent to which V-SFG probes interfacial water at sub-mM salt concns. As a consequence, V-SFG yields information about the ∼single monolayer interfacial region not only at very high ionic strength, where the surface charge is effectively screened, but also for pure water due to the particularly large screening length at this low ionic strength. At these low ionic strengths, the large screening lengths cause destructive interference between contributions in the surface region. A recently proposed theor. framework near-quant. describes our exptl. findings by considering only interference and screening. However, a comparison between NaCl and LiCl reveals ion specific effects in the screening efficiency of different electrolytes. Independent of electrolyte, the hydrogen bonding strength of water right at the interface is enhanced at high electrolyte concns.
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Gouy, M. Sur La Constitution de La Charge Électrique à La Surface d’un Électrolyte. J. Phys. Théorique Appliquée 1910, 9 (1), 457– 468, DOI: 10.1051/jphystap:019100090045700
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Chapman, D. L. A Contribution to the Theory of Electrocapillarity. London, Edinburgh Dublin Philos. Mag. J. Sci. 1913, 25 (148), 475– 481, DOI: 10.1080/14786440408634187
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Stern, O. Zur Theorie Der Elektrolytischen Doppelschicht. Zeitschrift fur Elektrochemie 1924, 30, 508– 516, DOI: 10.1002/bbpc.192400182
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The theory of the electrolytic double-layer
Stern, Otto
Zeitschrift fuer Elektrochemie und Angewandte Physikalische Chemie (1924), 30 (), 508-16CODEN: ZEAPAA; ISSN:0372-8323.
The electrolytic double-layer, metal-soln., may be considered a condenser one side of which is the surface of the electrode with a homogeneously distributed charge. On the liquid side of the boundary the elec. charge is partly concd. in the surface, while the remainder of it is situated in the electrolyte, the d. of the charge decreasing asymptotically towards zero. Based on this assumption, and for a simplified case, an equation is derived expressing the theories of polarization-capacity, of the electro-capillary curve, and of the electro-kinetic potential.
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Grahame, D. C. The Electrical Double Layer and the Theory of Electrocapillarity. Chem. Rev. 1947, 41 (3), 441– 501, DOI: 10.1021/cr60130a002
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The electrical double layer and the theory of electro-capillarity
Grahame, David C.
Chemical Reviews (Washington, DC, United States) (1947), 41 (), 441-501CODEN: CHREAY; ISSN:0009-2665.
A review with 95 references.
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The Electric Double Layer. In Physics and Chemistry of Interfaces ; 2003; pp 42– 56. DOI: 10.1002/3527602313.ch4 .
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Bockris, J. O.; Devanathan, M. A. V. A. V; Müller, K. On the Structure of Charged Interfaces. Electrochemistry 1965, 274 (1356), 832– 863, DOI: 10.1016/B978-1-4831-9831-6.50068-0
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Uddin, M. M.; Azam, M. S.; Hore, D. K. Variable-Angle Surface Spectroscopy Reveals the Water Structure in the Stern Layer at Charged Aqueous Interfaces. J. Am. Chem. Soc. 2024, 146 (17), 11756– 11763, DOI: 10.1021/jacs.3c14836
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Sun, S.; Schaefer, J.; Backus, E. H. G.; Bonn, M. How Surface-Specific Is 2nd-Order Non-Linear Spectroscopy?. J. Chem. Phys. 2019, 151 (23), 230901, DOI: 10.1063/1.5129108
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How surface-specific is 2nd-order non-linear spectroscopy?
Sun, Shumei; Schaefer, Jan; Backus, Ellen H. G.; Bonn, Mischa
Journal of Chemical Physics (2019), 151 (23), 230901/1-230901/7CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
A review. Surfaces and interfaces play important roles in many processes and reactions and are therefore intensively studied, often with the aim of obtaining mol.-level information from just the interfacial layer. Generally, only the first few mol. layers next to the interface are relevant for the surface processes. In the past decades, 2nd-order nonlinear spectroscopies including sum-frequency generation and second harmonic generation have developed into powerful tools for obtaining molecularly specific insights into the interfacial region. These approaches have contributed substantially to our understanding of a wide range of phys. phenomena. However, along with their wide-ranging applications, it has been realized that the implied surface-specificity of these approaches may not always be warranted. Specifically, the bulk quadrupole contribution beyond the elec. dipole-approxn. for a system with a weak nonlinear interface signal, as well as the diffuse layer contribution at charged interfaces, could mask the surface information. In this perspective paper, we discuss the surface-specificity of 2nd-order nonlinear spectroscopy, esp. considering these two contributions. (c) 2019 American Institute of Physics.
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Rehl, B.; Gibbs, J. M. Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability. J. Phys. Chem. Lett. 2021, 12 (11), 2854– 2864, DOI: 10.1021/acs.jpclett.0c03565
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Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability
Rehl, Benjamin; Gibbs, Julianne M.
Journal of Physical Chemistry Letters (2021), 12 (11), 2854-2864CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Isolating the hydrogen-bonding structure of water immediately at the surface is challenging, even with surface-specific techniques like sum-frequency generation (SFG), because of the presence of aligned water further away in the diffuse layer. Here, we combine zeta potential and SFG intensity measurements with the max. entropy method referenced to reported phase-sensitive SFG and second-harmonic generation results to deconvolute the SFG spectral contributions of the surface waters from those in the diffuse layer. Deconvolution reveals that at very low ionic strength, the surface water structure is similar to that of a neutral silica surface near the point-of-zero-charge with waters in different hydrogen-bonding environments oriented in opposite directions. This similarity suggests that the known metastability of silica colloids against aggregation under both conditions could arise from this distinct surface water structure. Upon the addn. of salt, significant restructuring of water is obsd., leading to a net decrease in order at the surface.
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DeWalt-Kerian, E. L.; Kim, S.; Azam, M. S.; Zeng, H.; Liu, Q.; Gibbs, J. M. PH-Dependent Inversion of Hofmeister Trends in the Water Structure of the Electrical Double Layer. J. Phys. Chem. Lett. 2017, 8 (13), 2855– 2861, DOI: 10.1021/acs.jpclett.7b01005
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pH-Dependent Inversion of Hofmeister Trends in the Water Structure of the Electrical Double Layer
DeWalt-Kerian, Emma L.; Kim, Sun; Azam, Md. Shafiul; Zeng, Hongbo; Liu, Qingxia; Gibbs, Julianne M.
Journal of Physical Chemistry Letters (2017), 8 (13), 2855-2861CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Specific ion effects (SIEs) are known to influence the acid/base behavior of silica and the interfacial structure of water, yet evidence of the effect of pH on SIEs is lacking. Here broadband vibrational sum frequency generation (SFG) spectroscopy was used to study SIEs on the water structure at the elec. double layer (EDL) of silica as a function of pH and monovalent cation identity from pH 2-12 at 0.5 M salt concn. SFG results indicate a direct Hofmeister series of cation adsorption at pH 8 (Li+ < Na+ < K+ < Cs+), with an inversion in this series occurring at pH > 10. In addn., an inversion in SFG intensity trends also occurred at pH < 6, which was attributed to contributions from asym. cation hydration and EDL overcharging. The highly pH-dependent SIEs for silica/water have implications for EDL models that often assume pH-independent parameters.
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Advincula, X. R.; Backus, E. H. G.; Bonn, M.; Cox, S. J.; Diebold, U.; Fellows, A.; Finney, A. R.; Goel, G.; Hedley, J.; Jiang, Y. Electrified/Charged Aqueous Interfaces: General Discussion. Faraday Discuss. 2024, 249 (0), 381– 407, DOI: 10.1039/D3FD90065G
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Ohno, P. E.; Saslow, S. A.; Wang, H.; Geiger, F. M.; Eisenthal, K. B. Phase-Referenced Nonlinear Spectroscopy of the α-Quartz/Water Interface. Nat. Commun. 2016, 7 (1), 13587, DOI: 10.1038/ncomms13587
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Phase-referenced nonlinear spectroscopy of the α-quartz/water interface
Ohno, Paul E.; Saslow, Sarah A.; Wang, Hong-fei; Geiger, Franz M.; Eisenthal, Kenneth B.
Nature Communications (2016), 7 (), 13587CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
Probing the polarization of water mols. at charged interfaces by second harmonic generation spectroscopy has been heretofore limited to isotropic materials. Here we report non-resonant nonlinear optical measurements at the interface of anisotropic z-cut α-quartz and water under conditions of dynamically changing ionic strength and bulk soln. pH. We find that the product of the third-order susceptibility and the interfacial potential, χ(3) × Φ(0), is given by (χ1(3)-iχ2(3)) × Φ(0), and that the interference between this product and the second-order susceptibility of bulk quartz depends on the rotation angle of α-quartz around the z axis. Our expts. show that this newly identified term, iχ(3) × Φ(0), which is out of phase from the surface terms, is of bulk origin. The possibility of internally phase referencing the interfacial response for the interfacial orientation anal. of species or materials in contact with α-quartz is discussed along with the implications for conditions of resonance enhancement.
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Ohno, P. E.; Wang, H.; Geiger, F. M. Second-Order Spectral Lineshapes from Charged Interfaces. Nat. Commun. 2017, 8 (1), 1032, DOI: 10.1038/s41467-017-01088-0
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Second-order spectral lineshapes from charged interfaces
Ohno Paul E; Geiger Franz M; Wang Hong-Fei
Nature communications (2017), 8 (1), 1032 ISSN:.
Second-order nonlinear spectroscopy has proven to be a powerful tool in elucidating key chemical and structural characteristics at a variety of interfaces. However, the presence of interfacial potentials may lead to complications regarding the interpretation of second harmonic and vibrational sum frequency generation responses from charged interfaces due to mixing of absorptive and dispersive contributions. Here, we examine by means of mathematical modeling how this interaction influences second-order spectral lineshapes. We discuss our findings in the context of reported nonlinear optical spectra obtained from charged water/air and solid/liquid interfaces and demonstrate the importance of accounting for the interfacial potential-dependent χ ((3)) term in interpreting lineshapes when seeking molecular information from charged interfaces using second-order spectroscopy.
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Wang, Y.; Nagata, Y.; Bonn, M. Substrate Effect on Charging of Electrified Graphene/Water Interfaces. Faraday Discuss. 2024, 249 (0), 303– 316, DOI: 10.1039/D3FD00107E
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Seki, T.; Yu, C. C.; Chiang, K. Y.; Tan, J.; Sun, S.; Ye, S.; Bonn, M.; Nagata, Y. Disentangling Sum-Frequency Generation Spectra of the Water Bending Mode at Charged Aqueous Interfaces. J. Phys. Chem. B 2021, 125 (25), 7060– 7067, DOI: 10.1021/acs.jpcb.1c03258
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Disentangling Sum-Frequency Generation Spectra of the Water Bending Mode at Charged Aqueous Interfaces
Seki, Takakazu; Yu, Chun-Chieh; Chiang, Kuo-Yang; Tan, Junjun; Sun, Shumei; Ye, Shuji; Bonn, Mischa; Nagata, Yuki
Journal of Physical Chemistry B (2021), 125 (25), 7060-7067CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
The origin of the sum-frequency generation (SFG) signal of the H2O bending mode was controversially debated in the past decade. Unveiling the origin of the signal is essential, because different assignments lead to different views on the mol. structure of interfacial H2O. Collinear heterodyne-detected SFG spectroscopy at the H2O-charged lipid interfaces was combined with systematic variation of the salt concn. The bending mode response is of a dipolar, rather than a quadrupolar, nature and allows one to disentangle the response of H2O in the Stern and the diffuse layers. While the diffuse layer response is identical for the oppositely charged surfaces, the Stern layer responses reflect interfacial H bonding. The H2O bending mode signal is a suitable probe for the structure of interfacial H2O.
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Mondal, J. A.; Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. Structure and Orientation of Water at Charged Lipid Monolayer/Water Interfaces Probed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy. J. Am. Chem. Soc. 2010, 132 (31), 10656– 10657, DOI: 10.1021/ja104327t
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Structure and Orientation of Water at Charged Lipid Monolayer/Water Interfaces Probed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy
Mondal, Jahur A.; Nihonyanagi, Satoshi; Yamaguchi, Shoichi; Tahara, Tahei
Journal of the American Chemical Society (2010), 132 (31), 10656-10657CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Cell membrane/water interfaces provide a unique environment for many biochem. reactions, and assocd. interfacial water is an integral part of such reactions. A mol. level understanding of the structure and orientation of water at lipid/water interfaces is required to realize the complex chem. at biointerfaces. Here, we report the heterodyne-detected vibrational sum frequency generation (HD-VSFG) studies of lipid monolayer/water interfaces. At charged lipid/water interfaces, the orientation of interfacial water is governed by the net charge on the lipid headgroup; at an anionic lipid/water interface, water is in the hydrogen-up orientation, and at the cationic lipid/water interface, water is in the hydrogen-down orientation. At the cationic and anionic lipid/water interfaces, interfacial water has comparable hydrogen bond strength, and it is analogous to the bulk water.
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Livingstone, R. A.; Nagata, Y.; Bonn, M.; Backus, E. H. G. Two Types of Water at the Water–Surfactant Interface Revealed by Time-Resolved Vibrational Spectroscopy. J. Am. Chem. Soc. 2015, 137 (47), 14912– 14919, DOI: 10.1021/jacs.5b07845
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Two Types of Water at the Water-Surfactant Interface Revealed by Time-Resolved Vibrational Spectroscopy
Livingstone, Ruth A.; Nagata, Yuki; Bonn, Mischa; Backus, Ellen H. G.
Journal of the American Chemical Society (2015), 137 (47), 14912-14919CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The surfactant sodium dodecyl sulfate (SDS) is widely used as a detergent for both domestic and industrial applications. It forms a self-assembled monolayer on the surface of water. The authors report a microscopic model for the interaction between the surfactant and water and between water mols. at the interface, revealed using static and time-resolved two-dimensional sum frequency generation spectroscopy. Two distinct subensembles of water in the presence of this neg. charged SDS surfactant have been identified: those close to the SDS headgroup having fairly isolated O-H groups, i.e., localized O-H stretch vibrations, and those whose O-H stretch vibrations are delocalized, i.e., shared between multiple O-H bonds. The two subensembles are coupled, with subpicosecond energy transfer occurring between them. This is markedly different from O-H bonds at the air-water interface, which are less heterogeneous, and indicates that the water mols. that interact with the surfactant headgroups have hydrogen-bonding properties different from those of water mols. interacting with the other water mols.
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Mondal, J. A.; Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. Three Distinct Water Structures at a Zwitterionic Lipid/Water Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation. J. Am. Chem. Soc. 2012, 134 (18), 7842– 7850, DOI: 10.1021/ja300658h
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Three Distinct Water Structures at a Zwitterionic Lipid/Water Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation
Mondal, Jahur A.; Nihonyanagi, Satoshi; Yamaguchi, Shoichi; Tahara, Tahei
Journal of the American Chemical Society (2012), 134 (18), 7842-7850CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Lipid/water interfaces and assocd. interfacial water are vital for various biochem. reactions, but the mol.-level understanding of their property is very limited. We investigated the water structure at a zwitterionic lipid, phosphatidylcholine (PC), monolayer/water interface using heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. Isotopically dild. water was utilized in the expts. to minimize the effect of intra/intermol. couplings. It was found that the OH stretch band in the Imχ(2) spectrum of the phosphatidylcholine/water interface exhibits a characteristic double-peaked feature. To interpret this peculiar spectrum of the zwitterionic lipid/water interface, Imχ(2) spectra of a zwitterionic surfactant/water interface and mixed lipid/water interfaces were measured. The Imχ(2) spectrum of the zwitterionic surfactant/water interface clearly shows both pos. and neg. bands in the OH stretch region, revealing that multiple water structures exist at the interface. At the mixed lipid/water interfaces, while gradually varying the fraction of the anionic and cationic lipids, we obsd. a drastic change in the Imχ(2) spectra in which spectral features similar to those of the anionic, zwitterionic, and cationic lipid/water interfaces appeared successively. These observations demonstrate that when the pos. and neg. charges coexist at the interface, the H-down-oriented water structure and H-up-oriented water structure appear in the vicinity of the resp. charged sites. In addn., it was found that a pos. Imχ(2) appears around 3600 cm-1 for all the monolayer interfaces examd., indicating weakly interacting water species existing in the hydrophobic region of the monolayer at the interface. On the basis of these results, we concluded that the characteristic Imχ(2) spectrum of the zwitterionic lipid/water interface arises from three different types of water existing at the interface: (1) the water assocd. with the neg. charged phosphate, which is strongly H-bonded and has a net H-up orientation, (2) the water around the pos. charged choline, which forms weaker H-bonds and has a net H-down orientation, and (3) the water weakly interacting with the hydrophobic region of the lipid, which has a net H-up orientation.
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Feng, R.; Guo, Y.; Lü, R.; Velarde, L.; Wang, H. Consistency in the Sum Frequency Generation Intensity and Phase Vibrational Spectra of the Air/Neat Water Interface. J. Phys. Chem. A 2011, 115 (23), 6015– 6027, DOI: 10.1021/jp110404h
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Consistency in the sum frequency generation intensity and phase vibrational spectra of the air/neat water interface
Feng, Ran-ran; Guo, Yuan; Lu, Rong; Velarde, Luis; Wang, Hong-fei
Journal of Physical Chemistry A (2011), 115 (23), 6015-6027CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
Substantial progress was made in the quant. understanding and interpretation of the H bonding and ordering structure of the air/H2O interface since the 1st sum-frequency generation vibrational spectroscopy (SFG-VS) measurement by Q. Du et al. (1993). There are still disagreements and controversies on the consistency between the different exptl. measurements, as well as in the theor. and computational results. One crit. problem lies in the lack of consistency between the SFG-VS intensity measurements and the recently developed SFG-VS phase spectra measurements of the neat air/H2O interface, which has inspired various theor. efforts. The reliability of the SFG-VS intensity spectra of the air/neat H2O interface is to be quant. examd., and possible sources of inaccuracies in the SFG-VS phase spectral measurement are to be discussed based on the nonresonant SHG phase measurements. Solid evidence is shown indicating that the SFG-VS intensity spectra from different labs. are now quant. converging and in agreement with each other. However, the possible inaccuracies and inconsistencies in the SFG-VS phase spectra measurements need to be carefully examd. against a properly cor. phase std. to take full advantage of this powerful exptl. tool.
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Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. Direct Evidence for Orientational Flip-Flop of Water Molecules at Charged Interfaces: A Heterodyne-Detected Vibrational Fum Frequency Generation Study. J. Chem. Phys. 2009, 130 (20), 204704 DOI: 10.1063/1.3135147
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Direct evidence for orientational flip-flop of water molecules at charged interfaces: A heterodyne-detected vibrational sum frequency generation study
Nihonyanagi, Satoshi; Yamaguchi, Shoichi; Tahara, Tahei
Journal of Chemical Physics (2009), 130 (20), 204704/1-204704/5CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
Complex χ(2) spectra of air/H2O interfaces in the presence of charged surfactants were measured by heterodyne-detected broadband vibrational sum frequency generation spectroscopy for the 1st time. In contrast to the neat H2O surface, the signs of χ(2) for two broad OH bands are the same in the presence of the charged surfactants. The obtained χ(2) spectra clearly showed flip-flop of the interfacial H2O mols. which is induced by the opposite charge of the head group of the surfactants. With the sign of β(2) theor. obtained, the abs. orientation, i.e., up/down orientation, of H2O mols. at the charged aq. surfaces was uniquely detd. by the relation between the sign of χ(2) and the mol. orientation angle. H2O mols. orient with their H up at the neg. charged aq. interface whereas their O up at the pos. charged aq. interface. (c) 2009 American Institute of Physics.
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Rehl, B.; Ma, E.; Parshotam, S.; DeWalt-Kerian, E. L.; Liu, T.; Geiger, F. M.; Gibbs, J. M. Water Structure in the Electrical Double Layer and the Contributions to the Total Interfacial Potential at Different Surface Charge Densities. J. Am. Chem. Soc. 2022, 144 (36), 16338– 16349, DOI: 10.1021/jacs.2c01830
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Water Structure in the Electrical Double Layer and the Contributions to the Total Interfacial Potential at Different Surface Charge Densities
Rehl, Benjamin; Ma, Emily; Parshotam, Shyam; DeWalt-Kerian, Emma L.; Liu, Tianli; Geiger, Franz M.; Gibbs, Julianne M.
Journal of the American Chemical Society (2022), 144 (36), 16338-16349CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The elec. double layer governs the processes of all charged surfaces in aq. solns.; however, elucidating the structure of the water mols. is challenging for even the most advanced spectroscopic techniques. Here, we present the individual Stern layer and diffuse layer OH stretching spectra at the silica/water interface in the presence of NaCl over a wide pH range using a combination of vibrational sum frequency generation spectroscopy, heterodyned second harmonic generation, and streaming potential measurements. We find that the Stern layer water mols. and diffuse layer water mols. respond differently to pH changes: unlike the diffuse layer, whose water mols. remain net-oriented in one direction, water mols. in the Stern layer flip their net orientation as the soln. pH is reduced from basic to acidic. We obtain an exptl. est. of the non-Gouy-Chapman (Stern) potential contribution to the total potential drop across the insulator/electrolyte interface and discuss it in the context of dipolar, quadrupolar, and higher order potential contributions that vary with the obsd. changes in the net orientation of water in the Stern layer. Our findings show that a purely Gouy-Chapman (Stern) view is insufficient to accurately describe the elec. double layer of aq. interfaces.
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Lagutchev, A.; Hambir, S. A.; Dlott, D. D. Nonresonant Background Suppression in Broadband Vibrational Sum-Frequency Generation Spectroscopy. J. Phys. Chem. C 2007, 111 (37), 13645– 13647, DOI: 10.1021/jp075391j
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Nonresonant Background Suppression in Broadband Vibrational Sum-Frequency Generation Spectroscopy
Lagutchev, Alexei; Hambir, Selezion A.; Dlott, Dana D.
Journal of Physical Chemistry C (2007), 111 (37), 13645-13647CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Suppression of the nonresonant background in vibrational sum-frequency generation (SFG) in the broadband multiplex configuration is achieved using a time-asym. pulse, created by passing a femtosecond pulse through a Fabry-Perot etalon, to temporally discriminate between the faster nonresonant and slower resonant contributions. A mixed time and frequency domain explanation of the SFG process is presented, and spectra with high time resolns. and high degrees of nonresonant background suppression were obtained using self-assembled alkanethiolate monolayers on Au.
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Backus, E. H. G.; Hosseinpour, S.; Ramanan, C.; Sun, S.; Schlegel, S. J.; Zelenka, M.; Jia, X.; Gebhard, M.; Devi, A.; Wang, H. I. Ultrafast Surface-Specific Spectroscopy of Water at a Photoexcited TiO2 Model Water-Splitting Photocatalyst. Angew. Chemie Int. Ed. 2024, 63 (8), e202312123 DOI: 10.1002/anie.202312123
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80
Wang, C.; Xing, Y.; Zhang, C.; Chen, P.; Xia, Y.; Li, J.; Gui, X. Water Structure at Coal/Water Interface: Insights from SFG Vibrational Spectroscopy and MD Simulation. Colloids Surfaces A Physicochem. Eng. Asp. 2024, 688, 133604 DOI: 10.1016/j.colsurfa.2024.133604
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81
Lee, S. E.; Carr, A. J.; Kumal, R. R.; Uysal, A. Monovalent Ion–Graphene Oxide Interactions Are Controlled by Carboxylic Acid Groups: Sum Frequency Generation Spectroscopy Studies. J. Chem. Phys. 2024, 160 (8), 84707, DOI: 10.1063/5.0189203
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82
Brown, J. B.; Qian, Y.; Wang, H.; Zhang, T.; Huang-Fu, Z.-C.; Rao, Y. Quantitative Signal Analysis of Sum-Frequency Scattering Experiments from Aerosol Surfaces. Anal. Chem. 2024, 96 (33), 13607– 13615, DOI: 10.1021/acs.analchem.4c02397
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83
Mapile, A. N.; LeRoy, M. A.; Fabrizio, K.; Scatena, L. F.; Brozek, C. K. The Surface of Colloidal Metal–Organic Framework Nanoparticles Revealed by Vibrational Sum Frequency Scattering Spectroscopy. ACS Nano 2024, 18 (20), 13406– 13414, DOI: 10.1021/acsnano.4c03758
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84
Jordan, C. J. C.; Coons, M. P.; Herbert, J. M.; Verlet, J. R. R. Spectroscopy and Dynamics of the Hydrated Electron at the Water/Air Interface. Nat. Commun. 2024, 15 (1), 182, DOI: 10.1038/s41467-023-44441-2
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85
Liu, C.; Qin, X.; Yu, C.; Guo, Y.; Zhang, Z. Probing the Adsorption Configuration of Methanol at a Charged Air/Aqueous Interface Using Nonlinear Spectroscopy. Phys. Chem. Chem. Phys. 2024, 26 (19), 14336– 14344, DOI: 10.1039/D3CP06317H
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86
Salafsky, J.; Johansson, P. K.; Abdelkader, E.; Otting, G. Ligand-Induced Conformational Changes in Protein Molecules Detected by Sum-Frequency Generation (SFG). Biophys. J. 2024, 123, 3678, DOI: 10.1016/j.bpj.2024.09.017
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87
Gahtori, P.; Gunwant, V.; Pandey, R. Probing the Influence of Hydrophobicity of Modified Gold Nanoparticles in Modulating the Lipid Surface Behavior Using Vibrational Sum Frequency Generation Spectroscopy. Langmuir 2024, 40 (40), 21211– 21221, DOI: 10.1021/acs.langmuir.4c02735
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88
Du, Q.; Superfine, R.; Freysz, E.; Shen, Y. R. Vibrational Spectroscopy of Water at the Vapor/Water Interface. Phys. Rev. Lett. 1993, 70 (15), 2313– 2316, DOI: 10.1103/PhysRevLett.70.2313
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88
Vibrational spectroscopy of water at the vapor/water interface
Du, Q.; Superfine, R.; Freysz, E.; Shen, Y. R.
Physical Review Letters (1993), 70 (15), 2313-16CODEN: PRLTAO; ISSN:0031-9007.
Using IR-visible sum-frequency generation the authors obtained the OH stretch vibrational spectra of water at the vapor/water interface. From the spectra, the authors deduce that more than 20% of the surface water mols. have one free OH projecting into the vapor. The spectrum is weakly temp. dependent from 10 to 80 °C. A monolayer of fatty alc. on water surface terminates the free OH groups and induces an icelike structure in the spectrum.
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89
Shen, Y. R.; Ostroverkhov, V. Sum-Frequency Vibrational Spectroscopy on Water Interfaces: Polar Orientation of Water Molecules at Interfaces. Chem. Rev. 2006, 106 (4), 1140– 1154, DOI: 10.1021/cr040377d
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89
Sum-frequency vibrational spectroscopy on water interfaces: Polar orientation of water molecules at interfaces
Shen, Yuen Ron; Ostroverkhov, Victor
Chemical Reviews (Washington, DC, United States) (2006), 106 (4), 1140-1154CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review. A review discusses the theory and exptl. arrangement of sum-frequency vibrational spectroscopy (SFVS), the SF vibrational spectra of various water interfaces, and a new SFVS technique that permits simultaneous measurements of amplitude and phase of the SF vibrational spectra for interfaces. It describes the common features of SF vibrational spectra of water interfaces, usual difficulties in interpretation of spectra, and the water interfacial structure.
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90
Sun, Q.; Guo, Y. Vibrational Sum Frequency Generation Spectroscopy of the Air/Water Interface. J. Mol. Liq. 2016, 213 (1–3), 28– 32, DOI: 10.1016/j.molliq.2015.11.004
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90
Vibrational sum frequency generation spectroscopy of the air/water interface
Sun, Q.; Guo, Y.
Journal of Molecular Liquids (2016), 213 (), 28-32CODEN: JMLIDT; ISSN:0167-7322. (Elsevier B.V.)
Vibrational sum frequency generation (SFG) spectroscopy is employed to investigate the water structure at an air/water interface. By obtaining the SFG intensity using different polarizations, it is reasonable to deconvolute the conventional SFG spectrum into five sub-bands, which are located at 3100, 3200, 3400, 3550 and 3700 cm- 1. In comparison with the Raman OH stretching bands of liq. water, we suggest that the five fitted sub-bands can be attributed to OH vibrations engaged in various local hydrogen-bonded networks, such as single donor-double acceptor (DAA), double donor-double acceptor (DDAA), single donor-single acceptor (DA), double donor-single acceptor (DDA), and free-OH vibrations, resp. Owing to the truncations of hydrogen bonds at the air/water interface, obvious structural differences between interfacial water and bulk water can be expected. For ambient water, both DA and DDAA are the primary hydrogen-bonded networks. By contrast, DA hydrogen bonding can be regarded as the primary structural motif at the air/water interface. Addnl., we suggest that the loss of DDAA hydrogen bonding at the air/water interface provides the phys. origin of surface tension, which can be applied to understand the formation of a water droplet.
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91
Ahmed, M.; Nihonyanagi, S.; Kundu, A.; Yamaguchi, S.; Tahara, T. Resolving the Controversy over Dipole versus Quadrupole Mechanism of Bend Vibration of Water in Vibrational Sum Frequency Generation Spectra. J. Phys. Chem. Lett. 2020, 11 (21), 9123– 9130, DOI: 10.1021/acs.jpclett.0c02644
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91
Resolving the Controversy over Dipole versus Quadrupole Mechanism of Bend Vibration of Water in Vibrational Sum Frequency Generation Spectra
Ahmed, Mohammed; Nihonyanagi, Satoshi; Kundu, Achintya; Yamaguchi, Shoichi; Tahara, Tahei
Journal of Physical Chemistry Letters (2020), 11 (21), 9123-9130CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Recently, there has been controversy over whether the HOH bend signal of water in the vibrational sum frequency generation (VSFG) spectrum arises from the conventional dipole mechanism or the quadrupole mechanism. Here, we show that the Im χ(2) (the imaginary part of the second-order nonlinear susceptibility) spectra of the HOH bend mode of water at oppositely charged monolayer/water interfaces all exhibit pos. bands, irresp. of the difference in the sign of the charge at the interface. Furthermore, it is found that the peak frequency of the HOH bend band substantially changes depending on the chem. structure of the charged headgroup located at the interface. These results demonstrate that the VSFG signal of the HOH bend vibration is generated from interfacial water with the interfacial quadrupole mechanism that is assocd. with the large field gradient of incident lights localized in a very thin region at the interface.
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Du, Q.; Freysz, E.; Shen, Y. R. Surface Vibrational Spectroscopic Studies of Hydrogen Bonding and Hydrophobicity. Science 1994, 264 (5160), 826– 828, DOI: 10.1126/science.264.5160.826
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92
Surface vibrational spectroscopic studies of hydrogen bonding and hydrophobicity
Du, Quan; Freysz, Eric; Shen, Y. Ron
Science (Washington, DC, United States) (1994), 264 (5160), 826-8CODEN: SCIEAS; ISSN:0036-8075.
Surface vibrational spectroscopy by sum-frequency generation was used to study hydrophobicity at the mol. level at various interfaces: water-surfactant-coated quartz, water-hexane, and water-air. In all cases, hydrophobicity was characterized by the appearance of dangling hydroxyl bonds on 25 percent of the surface water mols. At the water-quartz interface, packing restrictions force the water surface layer to have a more ordered, ice-like structure. A partly wettable water-quartz interface was also studied.
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Raymond, E. A.; Tarbuck, T. L.; Richmond, G. L. Isotopic Dilution Studies of the Vapor/Water Interface as Investigated by Vibrational Sum-Frequency Spectroscopy. J. Phys. Chem. B 2002, 106 (11), 2817– 2820, DOI: 10.1021/jp013967d
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93
Isotopic Dilution Studies of the Vapor/Water Interface as Investigated by Vibrational Sum-Frequency Spectroscopy
Raymond, E. A.; Tarbuck, T. L.; Richmond, G. L.
Journal of Physical Chemistry B (2002), 106 (11), 2817-2820CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
Vibrational sum-frequency spectroscopy (VSFS) studies of dil. HOD/H2O/D2O mixts. were used to study the H-bonding interactions at the vapor/H2O interface. The decoupled nature of the HOD vibrations produces a less complex OH stretching band than the highly coupled OH stretches of H2O, creating a clearer picture of the mol. environments present in the interfacial region. From the spectra of interfacial HOD, the frequency of the uncoupled donor OH stretching mode is similar to what is obsd. in bulk HOD studies, leading to the conclusion that the H bonding environment at the surface is similar to that found in liq. H2O. This conclusion is supported by the tetrahedrally coordinated region of the vapor/water VSF spectrum, where the predominant resonant intensity is centered around 3300 cm-1, consistent with bulk Raman and IR spectra of liq. H2O.
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Shiratori, K.; Morita, A. Theory of Quadrupole Contributions from Interface and Bulk in Second-Order Optical Processes. Bull. Chem. Soc. Jpn. 2012, 85 (10), 1061– 1076, DOI: 10.1246/bcsj.20120167
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95
Yamaguchi, S.; Shiratori, K.; Morita, A.; Tahara, T. Electric Quadrupole Contribution to the Nonresonant Background of Sum Frequency Generation at Air/Liquid Interfaces. J. Chem. Phys. 2011, 134 (18), 184705 DOI: 10.1063/1.3586811
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Electric quadrupole contribution to the nonresonant background of sum frequency generation at air/liquid interfaces
Yamaguchi, Shoichi; Shiratori, Kazuya; Morita, Akihiro; Tahara, Tahei
Journal of Chemical Physics (2011), 134 (18), 184705/1-184705/7CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
The authors study an elec. quadrupole contribution to sum frequency generation (SFG) at air/liq. interfaces in an electronically and vibrationally nonresonant condition. Heterodyne-detected electronic sum frequency generation spectroscopy of air/liq. interfaces reveals that nonresonant χ(2) (2nd-order nonlinear susceptibility) has a neg. sign and nearly the same value for all 8 liqs. studied. This result is rationalized from the theor. expressions of χ(2) with an elec. quadrupole contribution taken into account. The nonresonant background of SFG is predominantly due to interfacial nonlinear polarization having a quadrupole contribution. Although this nonlinear polarization is localized at the interface, it depends on quadrupolar χ(2) in the bulk as well as that at the interface. It means that the sign of nonresonant χ(2) bears no relation to the up vs. down alignment of interfacial mols., because nonresonant χ(2) has a quadrupolar origin. (c) 2011 American Institute of Physics.
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Shiratori, K.; Yamaguchi, S.; Tahara, T.; Morita, A. Computational Analysis of the Quadrupole Contribution in the Second-Harmonic Generation Spectroscopy for the Water/Vapor Interface. J. Chem. Phys. 2013, 138 (6), 64704, DOI: 10.1063/1.4790407
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97
Moll, C. J.; Versluis, J.; Bakker, H. J. Direct Evidence for a Surface and Bulk Specific Response in the Sum-Frequency Generation Spectrum of the Water Bend Vibration. Phys. Rev. Lett. 2021, 127 (11), 116001 DOI: 10.1103/PhysRevLett.127.116001
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Direct Evidence for a Surface and Bulk Specific Response in the Sum-Frequency Generation Spectrum of the Water Bend Vibration
Moll, C. J.; Versluis, J.; Bakker, H. J.
Physical Review Letters (2021), 127 (11), 116001CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
We study the bending mode of pure water and charged aq. surfaces using heterodyne-detected vibrational sum-frequency generation spectroscopy. We observe a low (1626 cm-1) and a high (1656 cm-1) frequency component that can be unambiguously assigned to an interfacial dipole and a bulk quadrupolar response, resp. We thus demonstrate that probing the bending mode provides structural and quant. information on both the surface and the bulk.
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Superfine, R.; Huang, J. Y.; Shen, Y. R. Phase Measurement for Surface Infrared–Visible Sum-Frequency Generation. Opt. Lett. 1990, 15 (22), 1276– 1278, DOI: 10.1364/OL.15.001276
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Superfine, R.; Huang, J. Y.; Shen, Y. R. Experimental Determination of the Sign of Molecular Dipole Moment Derivatives: An Infrared─Visible Sum Frequency Generation Absolute Phase Measurement Study. Chem. Phys. Lett. 1990, 172 (3), 303– 306, DOI: 10.1016/0009-2614(90)85408-5
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100
Shen, Y. R. Phase-Sensitive Sum-Frequency Spectroscopy. Annu. Rev. Phys. Chem. 2013, 64 (1), 129– 150, DOI: 10.1146/annurev-physchem-040412-110110
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101
Phase-sensitive sum-frequency spectroscopy
Shen, Y. R.
Annual Review of Physical Chemistry (2013), 64 (), 129-150CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews Inc.)
A review. Phase-sensitive sum-frequency spectroscopy (SFS) allows the complete measurement of the complex spectra of surface nonlinear response coeffs. Similar to linear spectroscopy, the spectrum of the imaginary part of a surface response coeff. directly characterizes surface resonances without complication. This newly developed technique has greatly enhanced the capability of surface SFS and provides many new research opportunities for surface science. This article describes the exptl. schemes and underlying theory for the technique and briefly reviews works that have clearly demonstrated its power.
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Yamaguchi, S.; Otosu, T. Progress in Phase-Sensitive Sum Frequency Generation Spectroscopy. Phys. Chem. Chem. Phys. 2021, 23 (34), 18253– 18267, DOI: 10.1039/D1CP01994E
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102
Progress in phase-sensitive sum frequency generation spectroscopy
Yamaguchi, Shoichi; Otosu, Takuhiro
Physical Chemistry Chemical Physics (2021), 23 (34), 18253-18267CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
A review. Sum frequency generation (SFG) spectroscopy is a unique and powerful tool for investigating surfaces and interfaces at the mol. level. Phase-sensitive SFG (PS-SFG) is an upgraded technique that can overcome the inherent drawbacks of conventional SFG. Here we review several methods of PS-SFG developed and reported in 1990-2020. We introduce how and by which group each PS-SFG method was designed and built in terms of interferometer implementation for optical heterodyne detection, with one exception of a recent numerical method that does not rely on interferometry. We also discuss how PS-SFG solved some typical problems for aq. interfaces that were once left open by conventional SFG. These problems and their solns. are good examples to demonstrate why PS-SFG is essential. In addn., we briefly note a few terminol. issues related with PS-SFG to avoid confusion.
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Xu, X.; Shen, Y. R.; Tian, C. Phase-Sensitive Sum Frequency Vibrational Spectroscopic Study of Air/Water Interfaces: H2O, D2O, and Diluted Isotopic Mixtures. J. Chem. Phys. 2019, 150 (14), 144701 DOI: 10.1063/1.5081135
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103
Phase-sensitive sum frequency vibrational spectroscopic study of air/water interfaces: H2O, D2O, and diluted isotopic mixtures
Xu, Xiaofan; Shen, Y. Ron; Tian, Chuanshan
Journal of Chemical Physics (2019), 150 (14), 144701/1-144701/5CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
Using phase-sensitive sum-frequency vibrational spectroscopy with a carefully chosen phase ref., we revisited the vibrational spectra of vapor/water interfaces of neat H2O and D2O as well as HDO in dild. isotopic mixts. Using z-cut quartz as the phase ref., with proper frequency scaling, the gross features of the OH and OD stretching spectra from H2O and D2O and from HDO in two isotopic mixts. look similar and agree with those reported earlier, but differences are also apparent. In particular, a weak pos. band at low frequencies, which has been asserted by mol. dynamic simulations but not detectable in the exptl. OH spectrum with pure H2O, is now visible in the OD spectrum. The differences must arise from the change of intermol. interaction of water mols. with their surrounding mols. upon exchange of the isotopes. (c) 2019 American Institute of Physics.
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Sathyanarayana, D. N. Vibrational Spectroscopy: Theory and Applications; New Age International (P) Ltd.: New Delhi, 2004.
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104
Lagunov, O.; Drenchev, N.; Chakarova, K.; Panayotov, D.; Hadjiivanov, K. Isotopic Labelling in Vibrational Spectroscopy: A Technique to Decipher the Structure of Surface Species. Top. Catal. 2017, 60 (19), 1486– 1495, DOI: 10.1007/s11244-017-0833-x
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105
Sun, S.; Tian, C.; Shen, Y. R. Surface Sum-Frequency Vibrational Spectroscopy of Nonpolar Media. Proc. Natl. Acad. Sci. U. S. A. 2015, 112 (19), 5883– 5887, DOI: 10.1073/pnas.1505438112
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106
Surface sum-frequency vibrational spectroscopy of nonpolar media
Sun, Shumei; Tian, Chuanshan; Shen, Y. Ron
Proceedings of the National Academy of Sciences of the United States of America (2015), 112 (19), 5883-5887CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
Sum-frequency generation spectroscopy is surface specific only if the bulk contribution to the signal is negligible. Negligible bulk contribution is, however, not necessarily true, even for media with inversion symmetry. The inevitable challenge is to find the surface spectrum in the presence of bulk contribution, part of which was believed to be inseparable from the surface contribution. Here, for nonpolar media, it is possible to sep. deduce surface and bulk spectra from combined phase-sensitive sum-frequency vibrational spectroscopic measurements in reflection and transmission. The study of benzene interfaces is presented as an example.
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Hore, D. K.; Tyrode, E. Probing Charged Aqueous Interfaces Near Critical Angles: Effect of Varying Coherence Length. J. Phys. Chem. C 2019, 123 (27), 16911– 16920, DOI: 10.1021/acs.jpcc.9b05256
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107
Probing Charged Aqueous Interfaces Near Critical Angles: Effect of Varying Coherence Length
Hore, Dennis K.; Tyrode, Eric
Journal of Physical Chemistry C (2019), 123 (27), 16911-16920CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Angle-resolved vibrational sum frequency generation expts. have been used to study the silica-water interface as a function of ionic strength. Well below the crit. angle, the sum frequency intensity increases up to 10-4 M NaCl and then drops. However, near the crit. angle, a plateau may be obsd. up to 10-4 M. We first demonstrate that this is a result of the interaction of a long Debye length at low ionic strength with a long coherence length near the crit. angles. In order to account for the behavior at the lowest concns. where surface potentials are typically large, it is necessary to consider an electrostatic potential that extends into the bulk aq. phase beyond the Debye-Huckel approxn. Because the extent of second- and third-order contributions to the nonlinear polarization can vary with ionic strength, but not with the angle of incidence, we perform a global fit to the exptl. data using our proposed model to ext. the relative magnitude of the two susceptibilities. The ionic strength dependence of this ratio points to the crit. nature of the silanol deprotonation and the development of surface charge and illustrates how surface water mols. respond. These results highlight the importance of varying the coherence length in order to probe the water structure at charged interfaces.
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Cai, C.; Azam, M. S.; Hore, D. K. Determining the Surface Potential of Charged Aqueous Interfaces Using Nonlinear Optical Methods. J. Phys. Chem. C 2021, 125 (45), 25307– 25315, DOI: 10.1021/acs.jpcc.1c07761
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108
Determining the Surface Potential of Charged Aqueous Interfaces Using Nonlinear Optical Methods
Cai, Canyu; Azam, Md. Shafiul; Hore, Dennis K.
Journal of Physical Chemistry C (2021), 125 (45), 25307-25315CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Second-order nonlinear optical techniques have recently been established as sensitive probes of charged interfaces through the nonlinear susceptibility of water and provide an attractive route to elucidate the surface potential. We discuss methods that have been proposed using electronic second-harmonic generation and vibrational sum-frequency generation. A detailed comparison is provided, using a unified notation and including a discussion of the assumptions that are either convenient or necessary in order to arrive at the surface potential. We then illustrate that, when using the full soln. of the Poisson-Boltzmann equation that is applicable to a wide range of surface potentials, several benefits may be realized. The first is that, when using off-resonance phase measurements, the surface potential can be detd. from the phase alone, without any addnl. information or calibrated intensity schemes. Next, we illustrate a scheme for surface potential measurement using intensity-only data off-resonance by changing the angle of incidence. Finally, we discuss the possibility of measurements that target the vibrational resonance with water and illustrate how the surface potential can be isolated from such data.
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Yu, C.-C.; Seki, T.; Wang, Y.; Bonn, M.; Nagata, Y. Polarization-Dependent Sum-Frequency Generation Spectroscopy for \AA{}ngstrom-Scale Depth Profiling of Molecules at Interfaces. Phys. Rev. Lett. 2022, 128 (22), 226001 DOI: 10.1103/PhysRevLett.128.226001
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Abstract
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Figure 1
Figure 1. Snapshot of the air–water interface and the corresponding depth-dependent density and second-order susceptibility calculated from molecular dynamics simulations. The depth-dependent susceptibility highlights the regions with anisotropic structure within the interfacial layer. GDS: Gibbs dividing surface.
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Figure 2
Figure 2. Imaginary parts of the heterodyned SFG spectra recorded in the SSP polarization combination for the air-H₂O interface in the H–O–H bending region, air–D₂O interface in the O–D stretching region, and OTS self-assembled monolayer on fused silica in the C–H stretching region. Reprinted in part with permission from ref (24). Copyright 2024 American Chemical Society.
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Figure 3
Figure 3. SFG from charged interfaces showing (a) a schematic of the Gouy-Chapman-Stern model describing the electrostatics of the interfacial region, with the depth evolution of the electric field and potential as well as the two-layer regime, and (b) imaginary parts of the heterodyned SFG spectra of the pure air–water interface (right axis) and that covered with a negatively charged surfactant monolayer (dihexadecylphosphate, DHP, left axis), thus with no added subphase electrolyte, both recorded in the SSP polarization combination. The sharp positive contribution at ∼2320 cm^–1 is not a feature of the sample, but arises from the spectral referencing to quartz. Reprinted in part with permission from ref (24). Copyright 2024 American Chemical Society.
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Figure 4
Figure 4. SFG spectra of the air–D₂O interface recorded in the SSP polarization combination showing (a) phase-resolved response split into real and imaginary parts, highlighting the significant nonresonant contribution (NR), and (b) intensity spectrum for the overall response (resonant + nonresonant) and the resonant-only response, having subtracted the H₂O NR spectrum. Reprinted in part with permission from ref (24). Copyright 2024 American Chemical Society.
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Figure 5
Figure 5. Schematic showing the spatial and mechanistic origins of the electric dipolar and quadrupolar signals to the overall SFG response. ID: interfacial dipole, IQ: interfacial quadrupole, QB: bulk quadrupole. (7)
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Figure 6
Figure 6. Amplitude and phase variations of the observed SFG signal due to depth. (a) Plots of the amplitude scaling and propagation phase shift for two different coherence lengths that are typical for SFG studies of aqueous interfaces. While the amplitude prefactor is only shown for two wavevector mismatches, as it independent of sign, the propagation phase is presented with Δk_z(2) taking either a positive or negative value, indicated by the two labels Δk_z(2+) and Δk_z(2−). (b) Resulting amplitude ratios and phase differences between the observed signals with these coherence lengths, showing the different cases for equal and opposite signs for the two Δk_z values, i.e., ϕ(1) – ϕ(2+) and ϕ(1) – ϕ(2−), respectively, for the phase differences (with the amplitude ratio being independent of sign). Further details on these plots can be found in the Supporting Information.
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Figure 7
Figure 7. Heterodyned SFG and DFG spectra recorded in the SSP polarization combination from the air–water interface covered with negatively charged surfactants (DHP), showing both the real and imaginary parts for a subphase NaCl salt concentration of (a) 10^–1 M and (b) 10^–4 M. Also shown are schematic Argand plots of the SFG and DFG phases at selected frequencies through the resonant line shape.
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Figure 8
Figure 8. Comparison of the amplitude (black) and phase (red) modulations of the different methods for accessing depth information from modulating the coherence length. The values for the plots are shown for two different anisotropic depths that are typical values for charged interfaces, with Debye lengths of 10 and 100 nm which occur for electrolyte concentrations of 1 mM and 10 μM, respectively. The plots are based on calculations described in the Supporting Information using values for the coherence lengths from the literature. (24,29,63) The red striped bars indicate the potential phase differences using these literature values; however such phase-resolved measurements having not been demonstrated. T/R: transmission/reflection, VA: variable-angle, MD: momentum (frequency)-dependent.
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Left to right: Martin Wolf, Álvaro Díaz Duque, Martin Thämer, and Alexander Fellows.
Alexander P. Fellows
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Alexander Fellows obtained his MA and MSci in Natural Sciences from the University of Cambridge, United Kingdom, after which he stayed on to complete a PhD in Chemistry. In 2022, he commenced postdoctoral research at the Fritz Haber Institute in Berlin. His research interests surround the connection between molecular structure and macroscopic behavior at interfaces, with a particular focus on biophysical systems.
Álvaro Díaz Duque
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Álvaro Díaz Duque obtained his undergraduate degree in Chemistry at the National University of Colombia and his MSc. in Chemistry at the Free University of Berlin. In 2021, he started his PhD at the Fritz Haber Institute in Berlin, in the field of second-order nonlinear vibrational spectroscopy of aqueous and charged aqueous interfaces. He is interested in understanding the dynamics and thermodynamics of processes governing electrified interfaces.
Vasileios Balos
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Vasileios Balos received his BSc and MSc in Applied Physics from the National Technical University of Athens, Greece, after which he pursued a PhD in Physical Chemistry at the Max Planck Institute for Polymer Research in Mainz, Germany, completing it in 2017 through the University of Amsterdam. He then joined the Fritz Haber Institute in Berlin as a postdoctoral researcher for four years. In 2021, he was awarded a Marie Skłodowska-Curie Individual Fellowship, which took him to IMDEA Nanoscience in Madrid, Spain. Since 2024, he has led his own independent research group there, focusing on molecular-level liquid dynamics and processes using advanced nonlinear bulk and surface-sensitive spectroscopic techniques.
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Left: Louis Lehman. Right: Roland R. Netz.
Louis Lehmann
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Louis Lehmann obtained a B.Sc. in Chemistry from the Technische Universität Berlin and a M.Sc. in Computational Sciences from the Freie Universität Berlin in 2021. He is currently pursuing a Ph.D. in Physics at the Freie Universität Berlin. His research focuses on the theoretical prediction of nonlinear spectroscopy in aqueous systems.
Roland R. Netz
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Roland Netz studied physics at the Technical University of Berlin and at MIT and received his Ph.D. in 1994 from the University of Cologne. After several postdoctoral positions he was appointed associate professor of physics at the LMU Munich in 2002 and full professor of physics at the TU Munich in 2004. Since 2011 he has held a chair in theoretical biosoft-matter physics at the Freie Universitä Berlin. His research focuses on the structure, dynamics and spectroscopy of water, proteins, polymers and charged systems using quantum, atomistic, and coarse-grained simulations as well as theoretical approaches.
Martin Wolf
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Martin Wolf received his PhD at Freie Universität Berlin, Germany, with Gerhard Ertl. After a postdoc in Austin, Texas, he set up a femtosecond surface spectroscopy laboratory at the Fritz Haber Institute of the MPG and was also visiting scientist at IBM Yorktown Heights. In 2000 he was appointed as full professor in experimental physics at Freie Universität Berlin and since 2008 he is director of the Department of Physical Chemistry of the Fritz Haber Institute. His research focuses on spatial-temporal dynamics of elementary excitations at interfaces.
Martin Thämer
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Martin Thämer studied Chemistry at the University of Münster, Germany and obtained his PhD in Physical Chemistry from the Technical University of Munich in 2012. After Postdoc positions at the Massachusetts Institute for Technology and the University of Chicago, he joined the Fritz-Haber-Institute in Berlin where he started a research group in 2019 that focusses on the investigation of molecular structures at interfaces using nonlinear spectroscopy and microscopy methods.
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  A review. This perspective is focused on surfaces and interfaces of cryst. ice studied with sum frequency generation (SFG) spectroscopy by several research groups within these twenty years. The intrinsic interface selectivity of SFG has already enabled one to det. structural aspects of the ice surfaces and interfaces through vibrational signatures of the OH stretch. Here we describe why SFG is suitable for ice surface science and how SFG has revealed the structure and dynamics of dangling bonds, quasi-liq. layer, hydrogen-bond network, and ordered protons at the ice surfaces, ice/substrate interfaces, and ice films on metal surfaces. In particular, we compare SFG spectra of ice reported by different groups to examine consistency between their expts. We elaborate current controversies on a few SFG studies of ice before presenting future outlook for ice surface science by SFG. Addnl. in the appendix we briefly explain some tech. issues that may help us resolve conflicts and make progress in the right way.
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  A review on the characterization of the surface mol. structure of polymeric materials using sum frequency generation vibrational spectroscopy. The in situ detection of the surface structures of some common polymers in air, surface segregation of small end groups, polymer surface restructuring in water and step-wise changed polymer blend surfaces are discussed. Studies of surface glass transition and surface structures modified by rubbing, plasma deposition, UV irradn., O ion and radical irradn. and wet etching also are discussed.
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  Johnson, C. M.; Baldelli, S. Vibrational Sum Frequency Spectroscopy Studies of the Influence of Solutes and Phospholipids at Vapor/Water Interfaces Relevant to Biological and Environmental Systems. Chem. Rev. 2014, 114 (17), 8416– 8446, DOI: 10.1021/cr4004902
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  Vibrational Sum Frequency Spectroscopy Studies of the Influence of Solutes and Phospholipids at Vapor/Water Interfaces Relevant to Biological and Environmental Systems
  Johnson, C. Magnus; Baldelli, Steven
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  A review. Vibrational sum frequency spectroscopy (VSFS) is a second-order nonlinear vibrational laser spectroscopy technique, with the unique property that it, under the elec. dipole approxn., is surface and interface specific. By their amphiphilic nature, surfactants have a detg. effect on water properties at the interface. The water surface is also influenced by sol. and less surface active mols. Solutes have more or less activity at the air/liq. interface and in these systems the water structure and the solute structure are mutually influenced. The weaker of the interactions between the solvent and solute is exemplified in the gas phase mols./water systems. Dissolved gases are also able to affect the water surface as the interface is a barrier they must pass in order to become solvated. At equil. the mols. are present in both the gas and soln. phase; however, the surface state might be stable enough to have a significant occupation. In this case the water surface structure has a large impact on the surface complexes. Examples of the effect of phospholipids and solutes, such as aldehydes, organosulfur compds., alcs., nitriles, acids, salts, and gases, are discussed.
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  Shultz, M. J.; Schnitzer, C.; Simonelli, D.; Baldelli, S. Sum Frequency Generation Spectroscopy of the Aqueous Interface: Ionic and Soluble Molecular Solutions. Int. Rev. Phys. Chem. 2000, 19 (1), 123– 153, DOI: 10.1080/014423500229882
  16
  Sum frequency generation spectroscopy of the aqueous interface. Ionic and soluble molecular solutions
  Shultz, Mary Jane; Schnitzer, Cheryl; Simonelli, Danielle; Baldelli, Steve
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  A review with 115 refs. The liq. interface of aq. solns. is of central importance to numerous phenomena from cloud processing of combustion generated oxides to corrosion degrdn. of structural materials to transport across cell membranes. Despite the importance of this interface, little mol.-level information was known about it prior to the last decade-and-a-half. Mol.-level information is important not only for a fundamental understanding of processes at interfaces, but also for predicting methods for diminishing deleterious effects. Recently, the non-linear spectroscopic method, sum frequency generation (SFG), was applied to the investigation of the structure of the liq. interface. This review focuses on the liq.-air interface of aq. solns. contg. sol., ionic species-H2SO4, HNO3, HCl, alkali sulfates and bisulfates, NaCl and NaNO3-as well as sol. mol. species-glycerol, H2SO4 and NH3. Ionic materials influence the structure of water at the interface through an elec. double layer which arises from the differential distribution of anions and cations near the interface. Due to the extreme size of the proton, the strongest field is generated by acidic materials. As the concn. of these ionic materials increases, ion pairs form diminishing the strength of the double layer. This enables the ion-pair complex to penetrate to the interface and either displace water or bind it into hydrated complexes. Sol. materials of lower surface tension partition to the interface and either displace water from the interface or bind water into hydrated complexes. In particular, the conjectured NH3-water complex on aq. solns. is obsd. and it is detd. to tilt 34-38° from the normal.
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17. 17
  Chen, Z. Investigating Buried Polymer Interfaces Using Sum Frequency Generation Vibrational Spectroscopy. Prog. Polym. Sci. 2010, 35 (11), 1376– 1402, DOI: 10.1016/j.progpolymsci.2010.07.003
  17
  Investigating buried polymer interfaces using sum frequency generation vibrational spectroscopy
  Chen, Zhan
  Progress in Polymer Science (2010), 35 (11), 1376-1402CODEN: PRPSB8; ISSN:0079-6700. (Elsevier Ltd.)
  A review. This paper reviews recent progress in the studies of buried polymer interfaces using sum frequency generation (SFG) vibrational spectroscopy. Both buried solid/liq. and solid/solid interfaces involving polymeric materials are discussed. SFG studies of polymer/water interfaces show that different polymers exhibit varied surface restructuring behavior in water, indicating the importance of probing polymer/water interfaces in situ. SFG has also been applied to the investigation of interfaces between polymers and other liqs. It has been found that mol. interactions at such polymer/liq. interfaces dictate interfacial polymer structures. The mol. structures of silane mols., which are widely used as adhesion promoters, have been investigated using SFG at buried polymer/silane and polymer/polymer interfaces, providing mol.-level understanding of polymer adhesion promotion. The mol. structures of polymer/solid interfaces have been examd. using SFG with several different exptl. geometries. These results have provided mol.-level information about polymer friction, adhesion, interfacial chem. reactions, interfacial electronic properties, and the structure of layer-by-layer deposited polymers. Such research has demonstrated that SFG is a powerful tool to probe buried interfaces involving polymeric materials, which are difficult to study by conventional surface sensitive anal. techniques.
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18. 18
  Yan, E. C. Y.; Fu, L.; Wang, Z.; Liu, W. Biological Macromolecules at Interfaces Probed by Chiral Vibrational Sum Frequency Generation Spectroscopy. Chem. Rev. 2014, 114 (17), 8471– 8498, DOI: 10.1021/cr4006044
  18
  Biological Macromolecules at Interfaces Probed by Chiral Vibrational Sum Frequency Generation Spectroscopy
  Yan, Elsa C. Y.; Fu, Li; Wang, Zhuguang; Liu, Wei
  Chemical Reviews (Washington, DC, United States) (2014), 114 (17), 8471-8498CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review of on the exptl. and theor. developments of chiral vibrational SFG spectroscopy and its applications in probing biomacromols. at interfaces during the past decade.
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19. 19
  Hosseinpour, S.; Roeters, S. J.; Bonn, M.; Peukert, W.; Woutersen, S.; Weidner, T. Structure and Dynamics of Interfacial Peptides and Proteins from Vibrational Sum-Frequency Generation Spectroscopy. Chem. Rev. 2020, 120 (7), 3420– 3465, DOI: 10.1021/acs.chemrev.9b00410
  19
  Structure and Dynamics of Interfacial Peptides and Proteins from Vibrational Sum-Frequency Generation Spectroscopy
  Hosseinpour, Saman; Roeters, Steven J.; Bonn, Mischa; Peukert, Wolfgang; Woutersen, Sander; Weidner, Tobias
  Chemical Reviews (Washington, DC, United States) (2020), 120 (7), 3420-3465CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. Proteins at interfaces play an important role in cell biol., immunol., bioengineering, and biomimetic material design. Many biol. processes are based on interfacial protein action, ranging from cellular communication to immune responses and the protein-driven mineralization of bone. Despite the importance of interfacial proteins, comparatively little is known about their structure. The std. methods for studying cryst. or soln.-phase proteins (X-ray diffraction and NMR) are not well suited for studying proteins at interfaces, and for these proteins, we still lack a corresponding technique that can provide the same level of structural resolns. This is not surprising in view of the challenges involved in probing the structure of proteins within monomol. films assembled at a very thin interface in situ. Vibrational sum-frequency generation (SFG) spectroscopy has the potential to overcome this "resoln. gap" and investigate the structure and dynamics of proteins at interfaces at the mol. level and with subpicosecond time resoln. While SFG studies were initially limited to simple model peptides, the past decade has seen a dramatic advancement of exptl. techniques and data anal. methods, which has made it possible to also study interfacial proteins and their folding, binding, orientation, hydration, and dynamics. In this review, we first explain the principles of protein SFG and the exptl. and theor. methods to measure and analyze protein SFG spectra. Then we give an extensive overview of the interfacial proteins studied thus far with SFG. We highlight representative examples to demonstrate recent advances in probing the structure of proteins at the interfaces of liqs., membranes, minerals, and synthetic materials.
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20. 20
  Lambert, A. G.; Davies, P. B.; Neivandt, D. J. Implementing the Theory of Sum Frequency Generation Vibrational Spectroscopy: A Tutorial Review. Appl. Spectrosc. Rev. 2005, 40 (2), 103– 145, DOI: 10.1081/ASR-200038326
  20
  Implementing the theory of sum frequency generation vibrational spectroscopy: a tutorial review
  Lambert, Alex G.; Davies, Paul B.; Neivandt, David J.
  Applied Spectroscopy Reviews (2005), 40 (2), 103-145CODEN: APSRBB; ISSN:0570-4928. (Taylor & Francis, Inc.)
  A review. The interfacial regions between bulk media, although often comprising only a fraction of the material present, are frequently the site of reactions and phenomena that dominate the macroscopic properties of the entire system. Spectroscopic investigations of such interfaces are often hampered by the lack of surface specificity of most available techniques. Sum frequency generation vibrational spectroscopy (SFS) is a non-linear optical technique which provides vibrational spectra of mols. solely at interfaces. The spectra may be analyzed to provide the polar orientation, mol. conformation, and av. tilt angle of the adsorbate to the surface normal. This article is aimed at newcomers to the field of SFS, and via a tutorial approach will present and develop the general sum frequency equations and then demonstrate how the fundamental theory elucidates the important exptl. properties of SFS.
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21. 21
  Shen, Y. R. Revisiting the Basic Theory of Sum-Frequency Generation. J. Chem. Phys. 2020, 153 (18), 180901 DOI: 10.1063/5.0030947
  21
  Revisiting the basic theory of sum-frequency generation
  Shen, Y. R.
  Journal of Chemical Physics (2020), 153 (18), 180901CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
  The basic theory of sum-frequency generation (SFG) is revisited. A rigorous derivation showing that linear optical transmission and reflection at an interface result from the interference of the incident wave and induced radiation wave in a medium is presented. The derivation is extended to SFG in a medium with a finite interface layer to see how SFG evolves. Detailed description on interface vs bulk and elec. dipole (ED) vs elec. quadrupole (EQ) contribution to SFG are provided with essentially no model dependence, putting the theory of SFG on a solid ground and removing possible existing confusions. Elec.-quadrupole contributions to SFG from the interface and bulk are discussed. It is seen that there is a relevant bulk EQ contribution intrinsically inseparable in measurement from the interface ED contribution but plays a major role among all EQ contributions; its importance relative to the ED part can only be judged by referring to the established ref. cases. (c) 2020 American Institute of Physics.
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22. 22
  Patterson, J. E. The Nonresonant Sum-Frequency Generation Response: The Not-so-Silent Partner. J. Chem. Phys. 2024, 161 (6), 60901, DOI: 10.1063/5.0221401
  There is no corresponding record for this reference.
23. 23
  Zhang, Y.; de Aguiar, H. B.; Hynes, J. T.; Laage, D. Water Structure, Dynamics, and Sum-Frequency Generation Spectra at Electrified Graphene Interfaces. J. Phys. Chem. Lett. 2020, 11 (3), 624– 631, DOI: 10.1021/acs.jpclett.9b02924
  23
  Water Structure, Dynamics, and Sum-Frequency Generation Spectra at Electrified Graphene Interfaces
  Zhang, Yiwei; de Aguiar, Hilton B.; Hynes, James T.; Laage, Damien
  Journal of Physical Chemistry Letters (2020), 11 (3), 624-631CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  The properties of water at an electrified graphene electrode are studied via classical mol. dynamics simulations with a const. potential approach. We show that the value of the applied electrode potential has dramatic effects on the structure and dynamics of interfacial water mols. While a pos. potential slows down the reorientational and translational dynamics of water, an increasing neg. potential first accelerates the interfacial water dynamics before a deceleration at very large magnitude potential values. Further, our spectroscopic calcns. indicate that the water rearrangements induced by electrified interfaces can be probed exptl. In particular, the calcd. water vibrational sum-frequency generation (SFG) spectra show that SFG specifically reports on the first two water layers at 0 V but that at larger magnitude applied potentials the resulting static field induces long-range contributions to the spectrum. Electrified graphene interfaces provide promising paradigm systems for comprehending both short- and long-range neighboring aq. system impacts.
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24. 24
  Fellows, A. P.; Duque, Á. D.; Balos, V.; Lehmann, L.; Netz, R. R.; Wolf, M.; Thämer, M. How Thick Is the Air–Water Interface?─A Direct Experimental Measurement of the Decay Length of the Interfacial Structural Anisotropy. Langmuir 2024, 40 (35), 18760– 18772, DOI: 10.1021/acs.langmuir.4c02571
  There is no corresponding record for this reference.
25. 25
  Morita, A.; Hynes, J. T. A Theoretical Analysis of the Sum Frequency Generation Spectrum of the Water Surface. Chem. Phys. 2000, 258 (2), 371– 390, DOI: 10.1016/S0301-0104(00)00127-0
  25
  A theoretical analysis of the sum frequency generation spectrum of the water surface
  Morita, A.; Hynes, J. T.
  Chemical Physics (2000), 258 (2-3), 371-390CODEN: CMPHC2; ISSN:0301-0104. (Elsevier Science B.V.)
  The present paper provides a theor. anal. of the sum frequency generation (SFG) spectrum of the water surface in the OH stretch mode frequency region based on ab initio MO theory and mol. dynamics simulation homol. dynamics simulation. The environmental effects on the normal modes of OH stretching vibration, their frequency shifts and hyperpolarizability are formulated and tested. The simulated SFG spectrum reproduces exptl. results quite well. The surface susceptibilities of the dangling bond and H-bonded bands have opposite signs in their imaginary parts, which indicates opposite OH directions at the surface assocd. with these bands. The former band turns out to be sensitive only to the top monolayer, and the latter band to a few top monolayers. Further anal. reveals that those 2 bands arise from quite different types of mol. orientations. The assignment of the SFG spectrum is also analyzed in terms of the sym./antisym. character of the OH stretching modes and the degree of mode delocalization.
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26. 26
  Ji, N.; Ostroverkhov, V.; Tian, C. S.; Shen, Y. R. Characterization of Vibrational Resonances of Water-Vapor Interfaces by Phase-Sensitive Sum-Frequency Spectroscopy. Phys. Rev. Lett. 2008, 100 (9), 96102, DOI: 10.1103/PhysRevLett.100.096102
  There is no corresponding record for this reference.
27. 27
  Nihonyanagi, S.; Ishiyama, T.; Lee, T.; Yamaguchi, S.; Bonn, M.; Morita, A.; Tahara, T. Unified Molecular View of the Air/Water Interface Based on Experimental and Theoretical χ(2) Spectra of an Isotopically Diluted Water Surface. J. Am. Chem. Soc. 2011, 133 (42), 16875– 16880, DOI: 10.1021/ja2053754
  27
  Unified Molecular View of the Air/Water Interface Based on Experimental and Theoretical χ(2) Spectra of an Isotopically Diluted Water Surface
  Nihonyanagi, Satoshi; Ishiyama, Tatsuya; Lee, Touk-kwan; Yamaguchi, Shoichi; Bonn, Mischa; Morita, Akihiro; Tahara, Tahei
  Journal of the American Chemical Society (2011), 133 (42), 16875-16880CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The energetically unfavorable termination of the hydrogen-bonded network of water mols. at the air/water interface causes mol. rearrangement to minimize the free energy. The long-standing question is how water minimizes the surface free energy. The combination of advanced, surface-specific nonlinear spectroscopy and theor. simulation provides new insights. The complex χ(2) spectra of isotopically dild. water surfaces obtained by heterodyne-detected sum frequency generation spectroscopy and mol. dynamics simulation show excellent agreement, assuring the validity of the microscopic picture given in the simulation. The present study indicates that there is no ice-like structure at the surface - in other words, there is no increase of tetrahedrally coordinated structure compared to the bulk - but that there are water pairs interacting with a strong hydrogen bond at the outermost surface. Intuitively, this can be considered a consequence of the lack of a hydrogen bond toward the upper gas phase, enhancing the lateral interaction at the boundary. This study also confirms that the major source of the isotope effect on the water χ(2) spectra is the intramol. anharmonic coupling, i.e., Fermi resonance.
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28. 28
  Inoue, K.; Ahmed, M.; Nihonyanagi, S.; Tahara, T. Reorientation-Induced Relaxation of Free OH at the Air/Water Interface Revealed by Ultrafast Heterodyne-Detected Nonlinear Spectroscopy. Nat. Commun. 2020, 11 (1), 5344, DOI: 10.1038/s41467-020-19143-8
  28
  Reorientation-induced relaxation of free OH at the air/water interface revealed by ultrafast heterodyne-detected nonlinear spectroscopy
  Inoue, Ken-ichi; Ahmed, Mohammed; Nihonyanagi, Satoshi; Tahara, Tahei
  Nature Communications (2020), 11 (1), 5344CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
  The uniqueness of water originates from its three-dimensional hydrogen-bond network, but this hydrogen-bond network is suddenly truncated at the interface and non-hydrogen-bonded OH (free OH) appears. Although this free OH is the most characteristic feature of interfacial water, the mol.-level understanding of its dynamic property is still limited due to the tech. difficulty. We study ultrafast vibrational relaxation dynamics of the free OH at the air/water interface using time-resolved heterodyne-detected vibrational sum frequency generation (TR-HD-VSFG) spectroscopy. With the use of singular value decompn. (SVD) anal., the vibrational relaxation (T1) times of the free OH at the neat H2O and isotopically-dild. water interfaces are detd. to be 0.87 ± 0.06 ps (neat H2O), 0.84 ± 0.09 ps (H2O/HOD/D2O = 1/2/1), and 0.88 ± 0.16 ps (H2O/HOD/D2O = 1/8/16). The absence of the isotope effect on the T1 time indicates that the main mechanism of the vibrational relaxation of the free OH is reorientation of the topmost water mols. The detd. sub-picosecond T1 time also suggests that the free OH reorients diffusively without the switching of the hydrogen-bond partner by the topmost water mol.
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29. 29
  Hsiao, Y.; Chou, T.-H.; Patra, A.; Wen, Y.-C. Momentum-Dependent Sum-Frequency Vibrational Spectroscopy of Bonded Interface Layer at Charged Water Interfaces. Sci. Adv. 2023, 9 (15), eadg2823 DOI: 10.1126/sciadv.adg2823
  There is no corresponding record for this reference.
30. 30
  Chiang, K.-Y.; Seki, T.; Yu, C. C.; Ohto, T.; Hunger, J.; Bonn, M.; Nagata, Y. The Dielectric Function Profile Across the Water Interface Through Surface-Specific Vibrational Spectroscopy and Simulations. Proc. Natl. Acad. Sci. U. S. A. 2022, 119 (36), e2204156119 DOI: 10.1073/pnas.2204156119
  There is no corresponding record for this reference.
31. 31
  Stiopkin, I. V.; Weeraman, C.; Pieniazek, P. A.; Shalhout, F. Y.; Skinner, J. L.; Benderskii, A. V. Hydrogen Bonding at the Water Surface Revealed by Isotopic Dilution Spectroscopy. Nature 2011, 474 (7350), 192– 195, DOI: 10.1038/nature10173
  31
  Hydrogen bonding at the water surface revealed by isotopic dilution spectroscopy
  Stiopkin, Igor V.; Weeraman, Champika; Pieniazek, Piotr A.; Shalhout, Fadel Y.; Skinner, James L.; Benderskii, Alexander V.
  Nature (London, United Kingdom) (2011), 474 (7350), 192-195CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
  The air-water interface is perhaps the most common liq. interface. It covers more than 70% of the Earth surface and strongly affects atm., aerosol and environmental chem. The air-water interface has also attracted much interest as a model system that allows rigorous tests of theory, with one fundamental question being just how thin it is. Theor. studies have suggested a surprisingly short healing length of about 3 Å (1 Å = 0.1 nm), with the bulk-phase properties of water recovered within the top few monolayers. However, direct exptl. evidence was elusive owing to the difficulty of depth-profiling the liq. surface on the Å scale. Most phys., chem., and biol. properties of water, such as viscosity, solvation, wetting and the hydrophobic effect, are detd. by its H-bond network. This can be probed by observing the lineshape of the OH-stretch mode, the frequency shift of which is related to the H-bond strength. Here we report a combined exptl. and theor. study of the air-water interface using surface-selective heterodyne-detected vibrational sum frequency spectroscopy to focus on the free OD transition found only in the topmost water layer. By using deuterated water and isotopic diln. to reveal the vibrational coupling mechanism, we find that the free OD stretch is affected only by intramol. coupling to the stretching of the other OD group on the same mol. The other OD stretch frequency indicates the strength of one of the first H bonds encountered at the surface; this is the donor H bond of the water mol. straddling the interface, which we find to be only slightly weaker than bulk-phase water H bonds. We infer from this observation a remarkably fast onset of bulk-phase behavior on crossing from the air into the water phase.
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32. 32
  Pieniazek, P. A.; Tainter, C. J.; Skinner, J. L. Interpretation of the Water Surface Vibrational Sum-Frequency Spectrum. J. Chem. Phys. 2011, 135 (4), 44701, DOI: 10.1063/1.3613623
  There is no corresponding record for this reference.
33. 33
  Pezzotti, S.; Galimberti, D. R.; Gaigeot, M.-P. 2D H-Bond Network as the Topmost Skin to the Air–Water Interface. J. Phys. Chem. Lett. 2017, 8 (13), 3133– 3141, DOI: 10.1021/acs.jpclett.7b01257
  33
  2D H-Bond Network as the Topmost Skin to the Air-Water Interface
  Pezzotti, Simone; Galimberti, Daria Ruth; Gaigeot, Marie-Pierre
  Journal of Physical Chemistry Letters (2017), 8 (13), 3133-3141CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  A description of the structure of water at the interface with the air (liq.-vapor LV interface) from state-of-the-art DFT-based mol. dynamics simulations is provided. A two-dimensional (2D) H-bond extended network has been identified and fully characterized, demonstrating that interfacial water is organized into a 2D sheet with H-bonds oriented parallel to the instantaneous surface and following its spatial and temporal oscillations. By analyzing the nonlinear vSFG (vibrational sum frequency generation) spectrum of the LV interface in terms of layer-by-layer signal, it is demonstrate that the 2D water sheet is solely responsible for the spectral signatures, hence providing the interfacial 3.5 Å thickness effectively probed in nonlinear interfacial spectroscopy. The 2D H-bond network unraveled here is the essential key to rationalize macroscopic properties of water-air interfaces, as demonstrated here for spectroscopy and the surface potential.
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34. 34
  Mukamel, S. Principles of Nonlinear Optical Spectroscopy; Oxford University Press: Oxford, 1995.
  There is no corresponding record for this reference.
35. 35
  Wang, H. F.; Gan, W.; Lu, R.; Rao, Y.; Wu, B. H. Quantitative Spectral and Orientational Analysis in Surface Sum Frequency Generation Vibrational Spectroscopy (SFG-VS). Int. Rev. Phys. Chem. 2005, 24 (2), 191– 256, DOI: 10.1080/01442350500225894
  35
  Quantitative spectral and orientational analysis in surface sum frequency generation vibrational spectroscopy (SFG-VS)
  Wang, Hong-Fei; Gan, Wei; Lu, Rong; Rao, Yi; Wu, Bao-Hua
  International Reviews in Physical Chemistry (2005), 24 (2), 191-256CODEN: IRPCDL; ISSN:0144-235X. (Taylor & Francis Ltd.)
  A review. Sum frequency generation vibrational spectroscopy (SFG-VS) was proven to be a uniquely effective spectroscopic technique in the study of mol. structure and conformations, as well as the dynamics of mol. interfaces. The ability to apply SFG-VS to complex mol. interfaces was limited by the ability to abstr. quant. information from SFG-VS expts. The authors try to make assessments of the limitations, issues and techniques as well as methodols. in quant. orientational and spectral anal. with SFG-VS. Based on these assessments, the authors also try to summarize recent developments in methodologies on quant. orientational and spectral anal. in SFG-VS, and their applications to detailed anal. of SFG-VS data of various vapor/neat liq. interfaces. A rigorous formulation of the polarization null angle (PNA) method is given for accurate detn. of the orientational parameter D = 〈cos θ 〉/〈cos3 θ〉, and comparison between the PNA method with the commonly used polarization intensity ratio (PIR) method is discussed. The polarization and incident angle dependencies of the SFG-VS intensity are reviewed, in the light of how exptl. arrangements can be optimized to effectively abstr. crucial information from the SFG-VS expts. The values and models of the local field factors in the mol. layers are discussed. To examine the validity and limitations of the bond polarizability deriv. model, the general expressions for mol. hyperpolarizability tensors and their expression with the bond polarizability deriv. model for C3v, C2v and C∞v mol. groups are given in the 2 appendixes. The bond polarizability deriv. model can quant. describe many aspects of the intensities obsd. in the SFG-VS spectrum of the vapor/neat liq. interfaces in different polarizations. Using the polarization anal. in SFG-VS, polarization selection rules or guidelines are developed for assignment of the SFG-VS spectrum. Using the selection rules, SFG-VS spectra of vapor/diol, and vapor/n-normal alc. (n ∼ 1-8) interfaces are assigned, and some of the ambiguity and confusion, as well as their implications in previous IR and Raman assignment, are duly discussed. The ability to assign a SFG-VS spectrum using the polarization selection rules makes SFG-VS not only an effective and useful vibrational spectroscopy technique for interface studies, but also a complementary vibrational spectroscopy method in general condensed phase studies. These developments will put quant. orientational and spectral anal. in SFG-VS on a more solid foundation. The formulations, concepts and issues discussed in this review are expected to find broad applications for studies on mol. interfaces in the future.
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36. 36
  Balos, V.; Garling, T.; Duque, A. D.; John, B.; Wolf, M.; Thämer, M. Phase-Sensitive Vibrational Sum and Difference Frequency-Generation Spectroscopy Enabling Nanometer-Depth Profiling at Interfaces. J. Phys. Chem. C 2022, 126 (26), 10818– 10832, DOI: 10.1021/acs.jpcc.2c01324
  There is no corresponding record for this reference.
37. 37
  Fellows, A. P.; Balos, V.; John, B.; Díaz Duque, Á.; Wolf, M.; Thämer, M. Obtaining Extended Insight into Molecular Systems by Probing Multiple Pathways in Second-Order Nonlinear Spectroscopy. J. Chem. Phys. 2023, 159 (16), 164201 DOI: 10.1063/5.0169534
  There is no corresponding record for this reference.
38. 38
  Gonella, G.; Lütgebaucks, C.; de Beer, A. G. F.; Roke, S. Second Harmonic and Sum-Frequency Generation from Aqueous Interfaces Is Modulated by Interference. J. Phys. Chem. C 2016, 120 (17), 9165– 9173, DOI: 10.1021/acs.jpcc.5b12453
  38
  Second Harmonic and Sum-Frequency Generation from Aqueous Interfaces Is Modulated by Interference
  Gonella, Grazia; Luetgebaucks, Cornelis; de Beer, Alex G. F.; Roke, Sylvie
  Journal of Physical Chemistry C (2016), 120 (17), 9165-9173CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  The interfacial region of aq. systems also known as the elec. double layer can be characterized on the mol. level with 2nd harmonic and sum-frequency generation (SHG/SFG). SHG and SFG are surface specific methods for isotropic liqs. Here, the authors model the SHG/SFG intensity in reflection, transmission, and scattering geometry taking into account the spatial variation of all fields. In the presence of a surface electrostatic field, interference effects, which originate from oriented H2O mols. on a length scale over which the potential decays, can strongly modify the probing depth as well as the expected intensity at ionic strengths <10-3 M. For reflection expts. this interference phenomenon leads to a significant redn. of the SHG/SFG intensity. Transmission mode expts. from aq. interfaces are hardly influenced. For SHG/SFG scattering expts. the same interference increases intensity and to modified scattering patterns. The predicted scattering patterns are verified exptl.
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39. 39
  Hu, X.-H.; Wei, F.; Wang, H.; Wang, H.-F. α-Quartz Crystal as Absolute Intensity and Phase Standard in Sum-Frequency Generation Vibrational Spectroscopy. J. Phys. Chem. C 2019, 123 (24), 15071– 15086, DOI: 10.1021/acs.jpcc.9b03202
  39
  α-Quartz Crystal as Absolute Intensity and Phase Standard in Sum-Frequency Generation Vibrational Spectroscopy
  Hu, Xiao-Hua; Wei, Feng; Wang, Hui; Wang, Hong-Fei
  Journal of Physical Chemistry C (2019), 123 (24), 15071-15086CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Quartz crystal was used as intensity and phase ref. std. in sum-frequency generation (SFG) and other nonlinear spectroscopy measurements. The authors examd. the issues regarding the most widely used z-cut α-quartz in SFG intensity and phase measurements, through systematic comparison of optical rotation, piezoelec., and SFG vibrational spectroscopy (SFG-VS) measurement results. For quartz crystal used as SFG-VS intensity std., the authors examd. the issue on the spectroscopic flatness of quartz crystal and Au film in the broad range of 1000-4000 cm-1, and at 2000-4000 cm-1, both quartz crystal and Au film are spectrally flat and are suitable for intensity ref. in the SFG-VS measurement. For quartz crystal being used as an abs. phase std., the detn. of the abs. direction of the quartz crystal is crucial, and in the past this was defined and detd. using piezoelec. measurement. The 1949 and 1945 IRE (Institute of Radio Engineers) stds. of defining the coordinates system based on the piezoelec. measurement for the quartz unit cell and crystal are often inconvenient in the applications to the nonlinear spectroscopy measurement and may cause confusion. Therefore, the authors proposed a new SFG/SHG (2nd-harmonic generation) convention to consistently define the quartz crystal coordinates system. Then, the internal heterodyne phase-resolved (IHPR) SFG-VS measurement of adsorbed mol. layers, e.g., OTS (octadecyltrichlorosilane), on the quartz crystal surface can be used to self-consistently det. the abs. azimuthal orientation direction of the quartz crystal, independent from the use of the piezoelec. measurement. In the end, the authors also examd. the issue regarding the abs. phase of the hyperpolarizability of various mol. groups, as well as quartz, under proper coordinates system. These results are useful for the future applications to the nonlinear spectroscopy, particularly the nonlinear spectroscopy for surface and interface studies.
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40. 40
  Thämer, M.; Garling, T.; Campen, R. K.; Wolf, M. Quantitative Determination of the Nonlinear Bulk and Surface Response from Alpha-Quartz Using Phase Sensitive SFG Spectroscopy. J. Chem. Phys. 2019, 151 (6), 064707, DOI: 10.1063/1.5109868
  There is no corresponding record for this reference.
41. 41
  Baldelli, S.; Schnitzer, C.; Shultz, M. J.; Campbell, D. J. Sum Frequency Generation Investigation of Water at the Surface of H2O/H2SO4 Binary Systems. J. Phys. Chem. B 1997, 101 (49), 10435– 10441, DOI: 10.1021/jp972376d
  41
  Sum Frequency Generation Investigation of Water at the Surface of H2O/H2SO4 Binary Systems
  Baldelli, Steve; Schnitzer, Cheryl; Shultz, Mary Jane; Campbell, D. J.
  Journal of Physical Chemistry B (1997), 101 (49), 10435-10441CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
  Structural data about water at the air/water interface of sulfuric acid solns. have been obtained with sum frequency generation spectroscopy. Sulfuric acid significantly affects the orientation of water at the interface. With as little as 0.01x (mole fraction) sulfuric acid, water is more highly oriented compared with the pure water surface. Surface water with one hydrogen free of hydrogen bonding, a "free OH", decreases in concn. as the sulfuric acid concn. is increased. Finally, there are no sulfuric acid free OH groups projecting out of the surface for solns. from 0.01x to 0.9x sulfuric acid. Observations of the structure of water are consistent with the fact that sulfuric acid/water solns. change from ionic in nature at low concns. (<0.01x) to ion pair complexes/hydrates at high concns. (>0.4x).
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42. 42
  Medders, G. R.; Paesani, F. Dissecting the Molecular Structure of the Air/Water Interface from Quantum Simulations of the Sum-Frequency Generation Spectrum. J. Am. Chem. Soc. 2016, 138 (11), 3912– 3919, DOI: 10.1021/jacs.6b00893
  42
  Dissecting the Molecular Structure of the Air/Water Interface from Quantum Simulations of the Sum-Frequency Generation Spectrum
  Medders, Gregory R.; Paesani, Francesco
  Journal of the American Chemical Society (2016), 138 (11), 3912-3919CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The mol. characterization of the air/water interface is a key step in understanding fundamental multiphase phenomena ranging from heterogeneous chem. processes in the atm. to the hydration of biomols. The apparent simplicity of the air/water interface, however, masks an underlying complexity assocd. with the dynamic nature of the water hydrogen-bond network that has so far hindered an unambiguous characterization of its microscopic properties. Here, we demonstrate that the application of quantum many-body mol. dynamics, which enables spectroscopically accurate simulations of water from the gas to the condensed phase, leads to a definitive mol.-level picture of the interface region. For the first time, excellent agreement is obtained between the simulated vibrational sum-frequency generation spectrum and the most recent state-of-the-art measurements, without requiring any empirical frequency shift or ad hoc scaling of the spectral intensity. A dissection of the spectral features demonstrates that a rigorous representation of nuclear quantum effects as well as of many-body energy and electrostatic contributions is necessary for a quant. reprodn. of the exptl. data. The accuracy of the simulations presented here indicates that quantum many-body mol. dynamics can enable predictive studies of aq. interfaces, which by complementing analogous exptl. measurements will provide unique mol. insights into multiphase and heterogeneous processes of relevance in chem., biol., materials science, and environmental research.
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43. 43
  Wang, J.; Chen, X.; Clarke, M. L.; Chen, Z. Detection of Chiral Sum Frequency Generation Vibrational Spectra of Proteins and Peptides at Interfaces in Situ. Proc. Natl. Acad. Sci. U. S. A. 2005, 102 (14), 4978– 4983, DOI: 10.1073/pnas.0501206102
  43
  Detection of chiral sum frequency generation vibrational spectra of proteins and peptides at interfaces in situ
  Wang, Jie; Chen, Xiaoyun; Clarke, Matthew L.; Chen, Zhan
  Proceedings of the National Academy of Sciences of the United States of America (2005), 102 (14), 4978-4983CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
  In this work, we demonstrate the feasibility to collect off-electronic resonance chiral sum frequency generation (SFG) vibrational spectra from interfacial proteins and peptides at the solid/liq. interface in situ. It is difficult to directly detect a chiral SFG vibrational spectrum from interfacial fibrinogen mols. By adopting an interference enhancement method, such a chiral SFG vibrational spectrum can be deduced from interference spectra between the normal achiral spectrum and the chiral spectrum. We found that the chiral SFG vibrational spectrum of interfacial fibrinogen was mainly contributed by the β-sheet structure. For a β-sheet peptide tachyplesin I, which may be quite ordered at the solid/liq. interface, chiral SFG vibrational spectra can be collected directly. We believe that these chiral signals are mainly contributed by elec. dipole contributions, which can dominate the chiroptical responses of uniaxial systems. For the first time, to our knowledge, this work indicates that the off-electronic resonance SFG technique is sensitive enough to collect chiral SFG vibrational spectra of interfacial proteins and peptides, providing more structural information to elucidate interfacial protein and peptide structures.
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44. 44
  Johnson, C. M.; Tyrode, E. Study of the Adsorption of Sodium Dodecyl Sulfate (SDS) at the Air/Water Interface: Targeting the Sulfate Headgroup Using Vibrational Sum Frequency Spectroscopy. Phys. Chem. Chem. Phys. 2005, 7 (13), 2635– 2640, DOI: 10.1039/b505219j
  44
  Study of the adsorption of sodium dodecyl sulfate (SDS) at the air/water interface: targeting the sulfate headgroup using vibrational sum frequency spectroscopy
  Johnson, C. Magnus; Tyrode, Eric
  Physical Chemistry Chemical Physics (2005), 7 (13), 2635-2640CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  The surface sensitive technique vibrational sum frequency spectroscopy (VSFS), was used to study the adsorption behavior of SDS to the liq./vapor interface of aq. solns., specifically targeting the sulfate headgroup stretches. In the spectral region extending from 980 to 1850 cm-1, only the vibrations due to the SO3 group were detectable. The fitted amplitudes for the sym. SO3 stretch obsd. at 1070 cm-1 for the polarization combinations ssp and ppp, were seen to follow the adsorption isotherm calcd. from surface tension measurements. The orientation of the sulfate headgroup in the concn. range spanning from 1.0 mM to above the crit. micellar concn. (c.m.c.) was obsd. to remain const. within exptl. error, with the pseudo-C3 axis close to the surface normal. Furthermore, the effect of increasing amts. of NaCl at SDS concns. above c.m.c. was also studied, showing an increase of ≈12% in the fitted amplitude for the sym. SO3 stretch when increasing the ionic strength from 0-300 mM NaCl. Interestingly, the orientation of the SDS headgroup was also obsd. to remain const. within this concn. range and identical to the case without NaCl.
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45. 45
  Bell, G. R.; Bain, C. D.; Ward, R. N. Sum-Frequency Vibrational Spectroscopy of Soluble Surfactants at the Air/Water Interface. J. Chem. Soc. Faraday Trans. 1996, 92 (4), 515– 523, DOI: 10.1039/ft9969200515
  45
  Sum-frequency vibrational spectroscopy of soluble surfactants at the air/water interface
  Bell, Graham R.; Bain, Colin D.; Ward, Robert N.
  Journal of the Chemical Society, Faraday Transactions (1996), 92 (4), 515-23CODEN: JCFTEV; ISSN:0956-5000. (Royal Society of Chemistry)
  Vibrational spectra of seven surfactants adsorbed at the surface of aq. solns. have been obtained by IR-VIS sum-frequency generation. From these spectra, the degree of conformational disorder and the angle of the terminal Me group are inferred. In general, the no. of gauche conformations increases as the area per chain increases. The angle of the Me group, which is an indicator of the tilt of the hydrocarbon chains, is not simply related to the area per chain. Comparison of surfactants with the same chain length and area per mol. shows that the structure of the chain region of the monolayer is sensitive to the nature of the head group and not just to the packing d. Quant. models to explain peak intensities in sum-frequency spectra of surfactants are discussed critically.
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46. 46
  Ma, G.; Allen, H. C. DPPC Langmuir Monolayer at the Air-Water Interface: Probing the Tail and Head Groups by Vibrational Sum Frequency Generation Spectroscopy. Langmuir 2006, 22 (12), 5341– 5349, DOI: 10.1021/la0535227
  46
  DPPC Langmuir Monolayer at the Air-Water Interface: Probing the Tail and Head Groups by Vibrational Sum Frequency Generation Spectroscopy
  Ma, Gang; Allen, Heather C.
  Langmuir (2006), 22 (12), 5341-5349CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
  Dipalmitoylphosphatidylcholine (DPPC) is the predominant lipid component in lung surfactant. In this study, the Langmuir monolayer of deuterated dipalmitoylphosphatidylcholine (DPPC-d62) in the liq.-expanded (LE) phase and the liq.-condensed (LC) phase has been investigated at the air-water interface with broad bandwidth sum frequency generation (BBSFG) spectroscopy combined with a Langmuir film balance. Four moieties of the DPPC mol. are probed by BBSFG: the terminal Me (CD3) groups of the tails, the methylene (CD2) groups of the tails, the choline methyls (CH3) in the headgroup, and the phosphate in the headgroup. BBSFG spectra of the four DPPC moieties provide information about chain conformation, chain orientation, headgroup orientation, and headgroup hydration. These results provide a comprehensive picture of the DPPC phase behavior at the air-water interface. In the LE phase, the DPPC hydrocarbon chains are conformationally disordered with a significant no. of gauche configurations. In the LC phase, the hydrocarbon chains are in an all-trans conformation and are tilted from the surface normal by 25°. In addn., the orientations of the tail terminal Me groups are found to remain nearly unchanged with the variation of surface area. Qual. anal. of the BBSFG spectra of the choline Me groups suggests that these Me groups are tilted but lie somewhat parallel to the surface plane in both the LE and LC phases. The dehydration of the phosphate headgroup due to the LE-LC phase transition is obsd. through the frequency blue shift of the phosphate sym. stretch in the fingerprint region. In addn., implications for lung surfactant function from this work are discussed.
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47. 47
  Wang, Y.; Seki, T.; Yu, X.; Yu, C.-C.; Chiang, K.-Y.; Domke, K. F.; Hunger, J.; Chen, Y.; Nagata, Y.; Bonn, M. Chemistry Governs Water Organization at a Graphene Electrode. Nature 2023, 615 (7950), E1– E2, DOI: 10.1038/s41586-022-05669-y
  There is no corresponding record for this reference.
48. 48
  Gonella, G.; Backus, E. H. G.; Nagata, Y.; Bonthuis, D. J.; Loche, P.; Schlaich, A.; Netz, R. R.; Kühnle, A.; McCrum, I. T.; Koper Water at Charged Interfaces. Nat. Rev. Chem. 2021, 5 (7), 466– 485, DOI: 10.1038/s41570-021-00293-2
  48
  Water at charged interfaces
  Gonella, Grazia; Backus, Ellen H. G.; Nagata, Yuki; Bonthuis, Douwe J.; Loche, Philip; Schlaich, Alexander; Netz, Roland R.; Kuehnle, Angelika; McCrum, Ian T.; Koper, Marc T. M.; Wolf, Martin; Winter, Bernd; Meijer, Gerard; Campen, R. Kramer; Bonn, Mischa
  Nature Reviews Chemistry (2021), 5 (7), 466-485CODEN: NRCAF7; ISSN:2397-3358. (Nature Portfolio)
  A review. The ubiquity of aq. solns. in contact with charged surfaces and the realization that the mol.-level details of water-surface interactions often det. interfacial functions and properties relevant in many natural processes have led to intensive research. Even so, many open questions remain regarding the mol. picture of the interfacial organization and preferential alignment of water mols., as well as the structure of water mols. and ion distributions at different charged interfaces. While water, solutes and charge are present in each of these systems, the substrate can range from living tissues to metals. This diversity in substrates has led to different communities considering each of these types of aq. interface. In this Review, by considering water in contact with metals, oxides and biomembranes, we show the essential similarity of these disparate systems. While in each case the classical mean-field theories can explain many macroscopic and mesoscopic observations, it soon becomes apparent that such theories fail to explain phenomena for which mol. properties are relevant, such as interfacial chem. conversion. We highlight the current knowledge and limitations in our understanding and end with a view towards future opportunities in the field.
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49. 49
  Geiger, F. M. Second Harmonic Generation, Sum Frequency Generation, and χ (3): Dissecting Environmental Interfaces with a Nonlinear Optical Swiss Army Knife. Annu. Rev. Phys. Chem. 2009, 60 (1), 61– 83, DOI: 10.1146/annurev.physchem.59.032607.093651
  49
  Second harmonic generation, sum frequency generation, and χ(3): dissecting environmental interfaces with a nonlinear optical Swiss army knife
  Geiger, Franz M.
  Annual Review of Physical Chemistry (2009), 60 (), 61-83CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews Inc.)
  This review discusses recent advances in the nonlinear optics of environmental interfaces. We discuss the quant. aspects of the label-free approaches presented here and demonstrate that nonlinear optics has now assumed the role of a Swiss Army knife that can be used to dissect, with mol. detail, the fundamental and practical aspects of environmental interfaces and heterogeneous geochem. environments. In this work, nonlinear optical methods are applied to complex org. mols., such as veterinary antibiotics, and to small inorg. anions and cations, such as nitrate and chromate, or cadmium, zinc, and manganese. The environmental implications of the thermodn., kinetic, spectroscopic, structural, and electrochem. data are discussed.
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50. 50
  Yan, E. C. Y.; Liu, Y.; Eisenthal, K. B. New Method for Determination of Surface Potential of Microscopic Particles by Second Harmonic Generation. J. Phys. Chem. B 1998, 102 (33), 6331– 6336, DOI: 10.1021/jp981335u
  50
  New Method for Determination of Surface Potential of Microscopic Particles by Second Harmonic Generation
  Yan, Elsa C. Y.; Liu, Yan; Eisenthal, Kenneth B.
  Journal of Physical Chemistry B (1998), 102 (33), 6331-6336CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
  A new noninvasive method for detg. the surface electrostatic potential and surface charge d. of microscopic particles using 2nd harmonic generation (SHG) is described. The surface electrostatic properties of 1.05 μm polystyrene sulfate spheres in aq. soln. and that of 0.22 μm oil droplets in aq. emulsions are obtained. Comparisons of the surface potentials obtained from SHG with the zeta potential obtained from electrophoresis are in excellent agreement with theor. predictions.
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51. 51
  Wang, H.-F. Sum Frequency Generation Vibrational Spectroscopy (SFG-VS) for Complex Molecular Surfaces and Interfaces: Spectral Lineshape Measurement and Analysis plus Some Controversial Issues. Prog. Surf. Sci. 2016, 91 (4), 155– 182, DOI: 10.1016/j.progsurf.2016.10.001
  51
  Sum frequency generation vibrational spectroscopy (SFG-VS) for complex molecular surfaces and interfaces: Spectral lineshape measurement and analysis plus some controversial issues
  Wang, Hong-Fei
  Progress in Surface Science (2016), 91 (4), 155-182CODEN: PSSFBP; ISSN:0079-6816. (Elsevier B.V.)
  Sum-frequency generation vibrational spectroscopy (SFG-VS) was first developed in the 1980s and it has been proven a uniquely sensitive and surface/interface selective spectroscopic probe for characterization of the structure, conformation and dynamics of mol. surfaces and interfaces. In recent years, there have been many progresses in the development of methodol. and instrumentation in the SFG-VS toolbox that have significantly broadened the application to complex mol. surfaces and interfaces. In this review, after presenting a unified view on the theory and methodol. focusing on the SFG-VS spectral lineshape, as well as the new opportunities in SFG-VS applications with such developments, some of the controversial issues that have been puzzling the community are discussed. The aim of this review is to present to the researchers and students interested in mol. surfaces and interfacial sciences up-to-date perspectives complementary to the existing textbooks and reviews on SFG-VS.
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52. 52
  Backus, E. H. G.; Schaefer, J.; Bonn, M. Probing the Mineral–Water Interface with Nonlinear Optical Spectroscopy. Angew. Chemie Int. Ed. 2021, 60 (19), 10482– 10501, DOI: 10.1002/anie.202003085
  There is no corresponding record for this reference.
53. 53
  Covert, P. A.; Hore, D. K. Geochemical Insight from Nonlinear Optical Studies of Mineral–Water Interfaces. Annu. Rev. Phys. Chem. 2016, 67, 233– 257, DOI: 10.1146/annurev-physchem-040215-112300
  53
  Geochemical Insight from Nonlinear Optical Studies of Mineral-Water Interfaces
  Covert, Paul A.; Hore, Dennis K.
  Annual Review of Physical Chemistry (2016), 67 (), 233-257CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews)
  The physics and chem. of mineral-water interfaces are complex, even in idealized systems. Our need to understand this complexity is driven by both pure and applied sciences, i.e., by the need for basic understanding of earth systems and for the knowledge to mitigate our influences upon them. The second-order nonlinear optical techniques of second-harmonic generation and sum-frequency generation spectroscopy have proven adept at probing these types of interfaces. This review focuses on the contributions to geochem. made by nonlinear optical methods. The types of questions probed have included a basic description of the structure adopted by water mols. at the mineral interface, how flow and porosity affect this structure, adsorption of trace metal and org. species, and dissoln. mechanisms. We also discuss directions and challenges that lie ahead and the outlook for the continued use of nonlinear optical methods for studies of mineral-water boundaries.
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54. 54
  Wen, Y.-C.; Zha, S.; Liu, X.; Yang, S.; Guo, P.; Shi, G.; Fang, H.; Shen, Y. R.; Tian, C. Unveiling Microscopic Structures of Charged Water Interfaces by Surface-Specific Vibrational Spectroscopy. Phys. Rev. Lett. 2016, 116 (1), 016101, DOI: 10.1103/PhysRevLett.116.016101
  54
  Unveiling microscopic structures of charged water interfaces by surface-specific vibrational spectroscopy
  Wen, Yu-Chieh; Zha, Shuai; Liu, Xing; Yang, Shanshan; Guo, Pan; Shia, Guosheng; Fang, Haiping; Shen, Y. Ron; Tian, Chuanshan
  Physical Review Letters (2016), 116 (1), 016101/1-016101/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
  A sum-frequency spectroscopy scheme is developed that allows the measurement of vibrational spectra of the interfacial mol. structure of charged water interfaces. The application of this scheme to a prototype lipid-aq. interface as a demonstration reveals an interfacial hydrogen-bonding water layer structure that responds sensitively to the charge state of the lipid headgroup and its interaction with specific ions. This novel technique provides unique opportunities to search for better understanding of electrochem. and biol. aq. interfaces at a deeper mol. level.
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55. 55
  Zhao, X.; Ong, S.; Eisenthal, K. B. Polarization of Water Molecules at a Charged Interface. Second Harmonic Studies of Charged Monolayers at the Air/Water Interface. Chem. Phys. Lett. 1993, 202 (6), 513– 520, DOI: 10.1016/0009-2614(93)90041-X
  55
  Polarization of water molecules at a charged interface. Second harmonic studies of charged monolayers at the air/water interface
  Zhao, Xiaolin; Ong, Shaowei; Eisenthal, Kenneth B.
  Chemical Physics Letters (1993), 202 (6), 513-20CODEN: CHPLBC; ISSN:0009-2614.
  The second harmonic generation from charged monolayers at air/water interfaces is linearly related to the interface elec. potential. This dependence is due to the polarization of water mols. in the electrostatic field of the charged monolayer. The obsd. linear dependence of the second harmonic field on the interface elec. potential serves as the basis of a proposed method for obtaining the interface potential, which it should be noted is a nonintrusive optical method. The Gouy-Chapman model is valid at the air/water charged interface up to at least a 1M total electrolyte concn.
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56. 56
  Schaefer, J.; Gonella, G.; Bonn, M.; Backus, E. H. G. Surface-Specific Vibrational Spectroscopy of the Water/Silica Interface: Screening and Interference. Phys. Chem. Chem. Phys. 2017, 19 (25), 16875– 16880, DOI: 10.1039/C7CP02251D
  56
  Surface-specific vibrational spectroscopy of the water/silica interface: screening and interference
  Schaefer, Jan; Gonella, Grazia; Bonn, Mischa; Backus, Ellen H. G.
  Physical Chemistry Chemical Physics (2017), 19 (25), 16875-16880CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  Surface-specific vibrational sum-frequency generation spectroscopy (V-SFG) is frequently used to obtain information about the mol. structure at charged interfaces. Here, we provide exptl. evidence that not only screening of surface charges but also interference limits the extent to which V-SFG probes interfacial water at sub-mM salt concns. As a consequence, V-SFG yields information about the ∼single monolayer interfacial region not only at very high ionic strength, where the surface charge is effectively screened, but also for pure water due to the particularly large screening length at this low ionic strength. At these low ionic strengths, the large screening lengths cause destructive interference between contributions in the surface region. A recently proposed theor. framework near-quant. describes our exptl. findings by considering only interference and screening. However, a comparison between NaCl and LiCl reveals ion specific effects in the screening efficiency of different electrolytes. Independent of electrolyte, the hydrogen bonding strength of water right at the interface is enhanced at high electrolyte concns.
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57. 57
  Gouy, M. Sur La Constitution de La Charge Électrique à La Surface d’un Électrolyte. J. Phys. Théorique Appliquée 1910, 9 (1), 457– 468, DOI: 10.1051/jphystap:019100090045700
  There is no corresponding record for this reference.
58. 58
  Chapman, D. L. A Contribution to the Theory of Electrocapillarity. London, Edinburgh Dublin Philos. Mag. J. Sci. 1913, 25 (148), 475– 481, DOI: 10.1080/14786440408634187
  There is no corresponding record for this reference.
59. 59
  Stern, O. Zur Theorie Der Elektrolytischen Doppelschicht. Zeitschrift fur Elektrochemie 1924, 30, 508– 516, DOI: 10.1002/bbpc.192400182
  59
  The theory of the electrolytic double-layer
  Stern, Otto
  Zeitschrift fuer Elektrochemie und Angewandte Physikalische Chemie (1924), 30 (), 508-16CODEN: ZEAPAA; ISSN:0372-8323.
  The electrolytic double-layer, metal-soln., may be considered a condenser one side of which is the surface of the electrode with a homogeneously distributed charge. On the liquid side of the boundary the elec. charge is partly concd. in the surface, while the remainder of it is situated in the electrolyte, the d. of the charge decreasing asymptotically towards zero. Based on this assumption, and for a simplified case, an equation is derived expressing the theories of polarization-capacity, of the electro-capillary curve, and of the electro-kinetic potential.
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60. 60
  Grahame, D. C. The Electrical Double Layer and the Theory of Electrocapillarity. Chem. Rev. 1947, 41 (3), 441– 501, DOI: 10.1021/cr60130a002
  60
  The electrical double layer and the theory of electro-capillarity
  Grahame, David C.
  Chemical Reviews (Washington, DC, United States) (1947), 41 (), 441-501CODEN: CHREAY; ISSN:0009-2665.
  A review with 95 references.
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61. 61
  The Electric Double Layer. In Physics and Chemistry of Interfaces ; 2003; pp 42– 56. DOI: 10.1002/3527602313.ch4 .
  There is no corresponding record for this reference.
62. 62
  Bockris, J. O.; Devanathan, M. A. V. A. V; Müller, K. On the Structure of Charged Interfaces. Electrochemistry 1965, 274 (1356), 832– 863, DOI: 10.1016/B978-1-4831-9831-6.50068-0
  There is no corresponding record for this reference.
63. 63
  Uddin, M. M.; Azam, M. S.; Hore, D. K. Variable-Angle Surface Spectroscopy Reveals the Water Structure in the Stern Layer at Charged Aqueous Interfaces. J. Am. Chem. Soc. 2024, 146 (17), 11756– 11763, DOI: 10.1021/jacs.3c14836
  There is no corresponding record for this reference.
64. 64
  Sun, S.; Schaefer, J.; Backus, E. H. G.; Bonn, M. How Surface-Specific Is 2nd-Order Non-Linear Spectroscopy?. J. Chem. Phys. 2019, 151 (23), 230901, DOI: 10.1063/1.5129108
  64
  How surface-specific is 2nd-order non-linear spectroscopy?
  Sun, Shumei; Schaefer, Jan; Backus, Ellen H. G.; Bonn, Mischa
  Journal of Chemical Physics (2019), 151 (23), 230901/1-230901/7CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
  A review. Surfaces and interfaces play important roles in many processes and reactions and are therefore intensively studied, often with the aim of obtaining mol.-level information from just the interfacial layer. Generally, only the first few mol. layers next to the interface are relevant for the surface processes. In the past decades, 2nd-order nonlinear spectroscopies including sum-frequency generation and second harmonic generation have developed into powerful tools for obtaining molecularly specific insights into the interfacial region. These approaches have contributed substantially to our understanding of a wide range of phys. phenomena. However, along with their wide-ranging applications, it has been realized that the implied surface-specificity of these approaches may not always be warranted. Specifically, the bulk quadrupole contribution beyond the elec. dipole-approxn. for a system with a weak nonlinear interface signal, as well as the diffuse layer contribution at charged interfaces, could mask the surface information. In this perspective paper, we discuss the surface-specificity of 2nd-order nonlinear spectroscopy, esp. considering these two contributions. (c) 2019 American Institute of Physics.
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65. 65
  Rehl, B.; Gibbs, J. M. Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability. J. Phys. Chem. Lett. 2021, 12 (11), 2854– 2864, DOI: 10.1021/acs.jpclett.0c03565
  65
  Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability
  Rehl, Benjamin; Gibbs, Julianne M.
  Journal of Physical Chemistry Letters (2021), 12 (11), 2854-2864CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Isolating the hydrogen-bonding structure of water immediately at the surface is challenging, even with surface-specific techniques like sum-frequency generation (SFG), because of the presence of aligned water further away in the diffuse layer. Here, we combine zeta potential and SFG intensity measurements with the max. entropy method referenced to reported phase-sensitive SFG and second-harmonic generation results to deconvolute the SFG spectral contributions of the surface waters from those in the diffuse layer. Deconvolution reveals that at very low ionic strength, the surface water structure is similar to that of a neutral silica surface near the point-of-zero-charge with waters in different hydrogen-bonding environments oriented in opposite directions. This similarity suggests that the known metastability of silica colloids against aggregation under both conditions could arise from this distinct surface water structure. Upon the addn. of salt, significant restructuring of water is obsd., leading to a net decrease in order at the surface.
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66. 66
  DeWalt-Kerian, E. L.; Kim, S.; Azam, M. S.; Zeng, H.; Liu, Q.; Gibbs, J. M. PH-Dependent Inversion of Hofmeister Trends in the Water Structure of the Electrical Double Layer. J. Phys. Chem. Lett. 2017, 8 (13), 2855– 2861, DOI: 10.1021/acs.jpclett.7b01005
  66
  pH-Dependent Inversion of Hofmeister Trends in the Water Structure of the Electrical Double Layer
  DeWalt-Kerian, Emma L.; Kim, Sun; Azam, Md. Shafiul; Zeng, Hongbo; Liu, Qingxia; Gibbs, Julianne M.
  Journal of Physical Chemistry Letters (2017), 8 (13), 2855-2861CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Specific ion effects (SIEs) are known to influence the acid/base behavior of silica and the interfacial structure of water, yet evidence of the effect of pH on SIEs is lacking. Here broadband vibrational sum frequency generation (SFG) spectroscopy was used to study SIEs on the water structure at the elec. double layer (EDL) of silica as a function of pH and monovalent cation identity from pH 2-12 at 0.5 M salt concn. SFG results indicate a direct Hofmeister series of cation adsorption at pH 8 (Li+ < Na+ < K+ < Cs+), with an inversion in this series occurring at pH > 10. In addn., an inversion in SFG intensity trends also occurred at pH < 6, which was attributed to contributions from asym. cation hydration and EDL overcharging. The highly pH-dependent SIEs for silica/water have implications for EDL models that often assume pH-independent parameters.
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67. 67
  Advincula, X. R.; Backus, E. H. G.; Bonn, M.; Cox, S. J.; Diebold, U.; Fellows, A.; Finney, A. R.; Goel, G.; Hedley, J.; Jiang, Y. Electrified/Charged Aqueous Interfaces: General Discussion. Faraday Discuss. 2024, 249 (0), 381– 407, DOI: 10.1039/D3FD90065G
  There is no corresponding record for this reference.
68. 68
  Ohno, P. E.; Saslow, S. A.; Wang, H.; Geiger, F. M.; Eisenthal, K. B. Phase-Referenced Nonlinear Spectroscopy of the α-Quartz/Water Interface. Nat. Commun. 2016, 7 (1), 13587, DOI: 10.1038/ncomms13587
  68
  Phase-referenced nonlinear spectroscopy of the α-quartz/water interface
  Ohno, Paul E.; Saslow, Sarah A.; Wang, Hong-fei; Geiger, Franz M.; Eisenthal, Kenneth B.
  Nature Communications (2016), 7 (), 13587CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
  Probing the polarization of water mols. at charged interfaces by second harmonic generation spectroscopy has been heretofore limited to isotropic materials. Here we report non-resonant nonlinear optical measurements at the interface of anisotropic z-cut α-quartz and water under conditions of dynamically changing ionic strength and bulk soln. pH. We find that the product of the third-order susceptibility and the interfacial potential, χ(3) × Φ(0), is given by (χ1(3)-iχ2(3)) × Φ(0), and that the interference between this product and the second-order susceptibility of bulk quartz depends on the rotation angle of α-quartz around the z axis. Our expts. show that this newly identified term, iχ(3) × Φ(0), which is out of phase from the surface terms, is of bulk origin. The possibility of internally phase referencing the interfacial response for the interfacial orientation anal. of species or materials in contact with α-quartz is discussed along with the implications for conditions of resonance enhancement.
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69. 69
  Ohno, P. E.; Wang, H.; Geiger, F. M. Second-Order Spectral Lineshapes from Charged Interfaces. Nat. Commun. 2017, 8 (1), 1032, DOI: 10.1038/s41467-017-01088-0
  69
  Second-order spectral lineshapes from charged interfaces
  Ohno Paul E; Geiger Franz M; Wang Hong-Fei
  Nature communications (2017), 8 (1), 1032 ISSN:.
  Second-order nonlinear spectroscopy has proven to be a powerful tool in elucidating key chemical and structural characteristics at a variety of interfaces. However, the presence of interfacial potentials may lead to complications regarding the interpretation of second harmonic and vibrational sum frequency generation responses from charged interfaces due to mixing of absorptive and dispersive contributions. Here, we examine by means of mathematical modeling how this interaction influences second-order spectral lineshapes. We discuss our findings in the context of reported nonlinear optical spectra obtained from charged water/air and solid/liquid interfaces and demonstrate the importance of accounting for the interfacial potential-dependent χ ((3)) term in interpreting lineshapes when seeking molecular information from charged interfaces using second-order spectroscopy.
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70. 70
  Wang, Y.; Nagata, Y.; Bonn, M. Substrate Effect on Charging of Electrified Graphene/Water Interfaces. Faraday Discuss. 2024, 249 (0), 303– 316, DOI: 10.1039/D3FD00107E
  There is no corresponding record for this reference.
71. 71
  Seki, T.; Yu, C. C.; Chiang, K. Y.; Tan, J.; Sun, S.; Ye, S.; Bonn, M.; Nagata, Y. Disentangling Sum-Frequency Generation Spectra of the Water Bending Mode at Charged Aqueous Interfaces. J. Phys. Chem. B 2021, 125 (25), 7060– 7067, DOI: 10.1021/acs.jpcb.1c03258
  71
  Disentangling Sum-Frequency Generation Spectra of the Water Bending Mode at Charged Aqueous Interfaces
  Seki, Takakazu; Yu, Chun-Chieh; Chiang, Kuo-Yang; Tan, Junjun; Sun, Shumei; Ye, Shuji; Bonn, Mischa; Nagata, Yuki
  Journal of Physical Chemistry B (2021), 125 (25), 7060-7067CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
  The origin of the sum-frequency generation (SFG) signal of the H2O bending mode was controversially debated in the past decade. Unveiling the origin of the signal is essential, because different assignments lead to different views on the mol. structure of interfacial H2O. Collinear heterodyne-detected SFG spectroscopy at the H2O-charged lipid interfaces was combined with systematic variation of the salt concn. The bending mode response is of a dipolar, rather than a quadrupolar, nature and allows one to disentangle the response of H2O in the Stern and the diffuse layers. While the diffuse layer response is identical for the oppositely charged surfaces, the Stern layer responses reflect interfacial H bonding. The H2O bending mode signal is a suitable probe for the structure of interfacial H2O.
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72. 72
  Mondal, J. A.; Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. Structure and Orientation of Water at Charged Lipid Monolayer/Water Interfaces Probed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy. J. Am. Chem. Soc. 2010, 132 (31), 10656– 10657, DOI: 10.1021/ja104327t
  72
  Structure and Orientation of Water at Charged Lipid Monolayer/Water Interfaces Probed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy
  Mondal, Jahur A.; Nihonyanagi, Satoshi; Yamaguchi, Shoichi; Tahara, Tahei
  Journal of the American Chemical Society (2010), 132 (31), 10656-10657CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Cell membrane/water interfaces provide a unique environment for many biochem. reactions, and assocd. interfacial water is an integral part of such reactions. A mol. level understanding of the structure and orientation of water at lipid/water interfaces is required to realize the complex chem. at biointerfaces. Here, we report the heterodyne-detected vibrational sum frequency generation (HD-VSFG) studies of lipid monolayer/water interfaces. At charged lipid/water interfaces, the orientation of interfacial water is governed by the net charge on the lipid headgroup; at an anionic lipid/water interface, water is in the hydrogen-up orientation, and at the cationic lipid/water interface, water is in the hydrogen-down orientation. At the cationic and anionic lipid/water interfaces, interfacial water has comparable hydrogen bond strength, and it is analogous to the bulk water.
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73. 73
  Livingstone, R. A.; Nagata, Y.; Bonn, M.; Backus, E. H. G. Two Types of Water at the Water–Surfactant Interface Revealed by Time-Resolved Vibrational Spectroscopy. J. Am. Chem. Soc. 2015, 137 (47), 14912– 14919, DOI: 10.1021/jacs.5b07845
  73
  Two Types of Water at the Water-Surfactant Interface Revealed by Time-Resolved Vibrational Spectroscopy
  Livingstone, Ruth A.; Nagata, Yuki; Bonn, Mischa; Backus, Ellen H. G.
  Journal of the American Chemical Society (2015), 137 (47), 14912-14919CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The surfactant sodium dodecyl sulfate (SDS) is widely used as a detergent for both domestic and industrial applications. It forms a self-assembled monolayer on the surface of water. The authors report a microscopic model for the interaction between the surfactant and water and between water mols. at the interface, revealed using static and time-resolved two-dimensional sum frequency generation spectroscopy. Two distinct subensembles of water in the presence of this neg. charged SDS surfactant have been identified: those close to the SDS headgroup having fairly isolated O-H groups, i.e., localized O-H stretch vibrations, and those whose O-H stretch vibrations are delocalized, i.e., shared between multiple O-H bonds. The two subensembles are coupled, with subpicosecond energy transfer occurring between them. This is markedly different from O-H bonds at the air-water interface, which are less heterogeneous, and indicates that the water mols. that interact with the surfactant headgroups have hydrogen-bonding properties different from those of water mols. interacting with the other water mols.
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74. 74
  Mondal, J. A.; Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. Three Distinct Water Structures at a Zwitterionic Lipid/Water Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation. J. Am. Chem. Soc. 2012, 134 (18), 7842– 7850, DOI: 10.1021/ja300658h
  74
  Three Distinct Water Structures at a Zwitterionic Lipid/Water Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation
  Mondal, Jahur A.; Nihonyanagi, Satoshi; Yamaguchi, Shoichi; Tahara, Tahei
  Journal of the American Chemical Society (2012), 134 (18), 7842-7850CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Lipid/water interfaces and assocd. interfacial water are vital for various biochem. reactions, but the mol.-level understanding of their property is very limited. We investigated the water structure at a zwitterionic lipid, phosphatidylcholine (PC), monolayer/water interface using heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. Isotopically dild. water was utilized in the expts. to minimize the effect of intra/intermol. couplings. It was found that the OH stretch band in the Imχ(2) spectrum of the phosphatidylcholine/water interface exhibits a characteristic double-peaked feature. To interpret this peculiar spectrum of the zwitterionic lipid/water interface, Imχ(2) spectra of a zwitterionic surfactant/water interface and mixed lipid/water interfaces were measured. The Imχ(2) spectrum of the zwitterionic surfactant/water interface clearly shows both pos. and neg. bands in the OH stretch region, revealing that multiple water structures exist at the interface. At the mixed lipid/water interfaces, while gradually varying the fraction of the anionic and cationic lipids, we obsd. a drastic change in the Imχ(2) spectra in which spectral features similar to those of the anionic, zwitterionic, and cationic lipid/water interfaces appeared successively. These observations demonstrate that when the pos. and neg. charges coexist at the interface, the H-down-oriented water structure and H-up-oriented water structure appear in the vicinity of the resp. charged sites. In addn., it was found that a pos. Imχ(2) appears around 3600 cm-1 for all the monolayer interfaces examd., indicating weakly interacting water species existing in the hydrophobic region of the monolayer at the interface. On the basis of these results, we concluded that the characteristic Imχ(2) spectrum of the zwitterionic lipid/water interface arises from three different types of water existing at the interface: (1) the water assocd. with the neg. charged phosphate, which is strongly H-bonded and has a net H-up orientation, (2) the water around the pos. charged choline, which forms weaker H-bonds and has a net H-down orientation, and (3) the water weakly interacting with the hydrophobic region of the lipid, which has a net H-up orientation.
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75. 75
  Feng, R.; Guo, Y.; Lü, R.; Velarde, L.; Wang, H. Consistency in the Sum Frequency Generation Intensity and Phase Vibrational Spectra of the Air/Neat Water Interface. J. Phys. Chem. A 2011, 115 (23), 6015– 6027, DOI: 10.1021/jp110404h
  75
  Consistency in the sum frequency generation intensity and phase vibrational spectra of the air/neat water interface
  Feng, Ran-ran; Guo, Yuan; Lu, Rong; Velarde, Luis; Wang, Hong-fei
  Journal of Physical Chemistry A (2011), 115 (23), 6015-6027CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  Substantial progress was made in the quant. understanding and interpretation of the H bonding and ordering structure of the air/H2O interface since the 1st sum-frequency generation vibrational spectroscopy (SFG-VS) measurement by Q. Du et al. (1993). There are still disagreements and controversies on the consistency between the different exptl. measurements, as well as in the theor. and computational results. One crit. problem lies in the lack of consistency between the SFG-VS intensity measurements and the recently developed SFG-VS phase spectra measurements of the neat air/H2O interface, which has inspired various theor. efforts. The reliability of the SFG-VS intensity spectra of the air/neat H2O interface is to be quant. examd., and possible sources of inaccuracies in the SFG-VS phase spectral measurement are to be discussed based on the nonresonant SHG phase measurements. Solid evidence is shown indicating that the SFG-VS intensity spectra from different labs. are now quant. converging and in agreement with each other. However, the possible inaccuracies and inconsistencies in the SFG-VS phase spectra measurements need to be carefully examd. against a properly cor. phase std. to take full advantage of this powerful exptl. tool.
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76. 76
  Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. Direct Evidence for Orientational Flip-Flop of Water Molecules at Charged Interfaces: A Heterodyne-Detected Vibrational Fum Frequency Generation Study. J. Chem. Phys. 2009, 130 (20), 204704 DOI: 10.1063/1.3135147
  76
  Direct evidence for orientational flip-flop of water molecules at charged interfaces: A heterodyne-detected vibrational sum frequency generation study
  Nihonyanagi, Satoshi; Yamaguchi, Shoichi; Tahara, Tahei
  Journal of Chemical Physics (2009), 130 (20), 204704/1-204704/5CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
  Complex χ(2) spectra of air/H2O interfaces in the presence of charged surfactants were measured by heterodyne-detected broadband vibrational sum frequency generation spectroscopy for the 1st time. In contrast to the neat H2O surface, the signs of χ(2) for two broad OH bands are the same in the presence of the charged surfactants. The obtained χ(2) spectra clearly showed flip-flop of the interfacial H2O mols. which is induced by the opposite charge of the head group of the surfactants. With the sign of β(2) theor. obtained, the abs. orientation, i.e., up/down orientation, of H2O mols. at the charged aq. surfaces was uniquely detd. by the relation between the sign of χ(2) and the mol. orientation angle. H2O mols. orient with their H up at the neg. charged aq. interface whereas their O up at the pos. charged aq. interface. (c) 2009 American Institute of Physics.
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77. 77
  Rehl, B.; Ma, E.; Parshotam, S.; DeWalt-Kerian, E. L.; Liu, T.; Geiger, F. M.; Gibbs, J. M. Water Structure in the Electrical Double Layer and the Contributions to the Total Interfacial Potential at Different Surface Charge Densities. J. Am. Chem. Soc. 2022, 144 (36), 16338– 16349, DOI: 10.1021/jacs.2c01830
  77
  Water Structure in the Electrical Double Layer and the Contributions to the Total Interfacial Potential at Different Surface Charge Densities
  Rehl, Benjamin; Ma, Emily; Parshotam, Shyam; DeWalt-Kerian, Emma L.; Liu, Tianli; Geiger, Franz M.; Gibbs, Julianne M.
  Journal of the American Chemical Society (2022), 144 (36), 16338-16349CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The elec. double layer governs the processes of all charged surfaces in aq. solns.; however, elucidating the structure of the water mols. is challenging for even the most advanced spectroscopic techniques. Here, we present the individual Stern layer and diffuse layer OH stretching spectra at the silica/water interface in the presence of NaCl over a wide pH range using a combination of vibrational sum frequency generation spectroscopy, heterodyned second harmonic generation, and streaming potential measurements. We find that the Stern layer water mols. and diffuse layer water mols. respond differently to pH changes: unlike the diffuse layer, whose water mols. remain net-oriented in one direction, water mols. in the Stern layer flip their net orientation as the soln. pH is reduced from basic to acidic. We obtain an exptl. est. of the non-Gouy-Chapman (Stern) potential contribution to the total potential drop across the insulator/electrolyte interface and discuss it in the context of dipolar, quadrupolar, and higher order potential contributions that vary with the obsd. changes in the net orientation of water in the Stern layer. Our findings show that a purely Gouy-Chapman (Stern) view is insufficient to accurately describe the elec. double layer of aq. interfaces.
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78. 78
  Lagutchev, A.; Hambir, S. A.; Dlott, D. D. Nonresonant Background Suppression in Broadband Vibrational Sum-Frequency Generation Spectroscopy. J. Phys. Chem. C 2007, 111 (37), 13645– 13647, DOI: 10.1021/jp075391j
  78
  Nonresonant Background Suppression in Broadband Vibrational Sum-Frequency Generation Spectroscopy
  Lagutchev, Alexei; Hambir, Selezion A.; Dlott, Dana D.
  Journal of Physical Chemistry C (2007), 111 (37), 13645-13647CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Suppression of the nonresonant background in vibrational sum-frequency generation (SFG) in the broadband multiplex configuration is achieved using a time-asym. pulse, created by passing a femtosecond pulse through a Fabry-Perot etalon, to temporally discriminate between the faster nonresonant and slower resonant contributions. A mixed time and frequency domain explanation of the SFG process is presented, and spectra with high time resolns. and high degrees of nonresonant background suppression were obtained using self-assembled alkanethiolate monolayers on Au.
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79. 79
  Backus, E. H. G.; Hosseinpour, S.; Ramanan, C.; Sun, S.; Schlegel, S. J.; Zelenka, M.; Jia, X.; Gebhard, M.; Devi, A.; Wang, H. I. Ultrafast Surface-Specific Spectroscopy of Water at a Photoexcited TiO2 Model Water-Splitting Photocatalyst. Angew. Chemie Int. Ed. 2024, 63 (8), e202312123 DOI: 10.1002/anie.202312123
  There is no corresponding record for this reference.
80. 80
  Wang, C.; Xing, Y.; Zhang, C.; Chen, P.; Xia, Y.; Li, J.; Gui, X. Water Structure at Coal/Water Interface: Insights from SFG Vibrational Spectroscopy and MD Simulation. Colloids Surfaces A Physicochem. Eng. Asp. 2024, 688, 133604 DOI: 10.1016/j.colsurfa.2024.133604
  There is no corresponding record for this reference.
81. 81
  Lee, S. E.; Carr, A. J.; Kumal, R. R.; Uysal, A. Monovalent Ion–Graphene Oxide Interactions Are Controlled by Carboxylic Acid Groups: Sum Frequency Generation Spectroscopy Studies. J. Chem. Phys. 2024, 160 (8), 84707, DOI: 10.1063/5.0189203
  There is no corresponding record for this reference.
82. 82
  Brown, J. B.; Qian, Y.; Wang, H.; Zhang, T.; Huang-Fu, Z.-C.; Rao, Y. Quantitative Signal Analysis of Sum-Frequency Scattering Experiments from Aerosol Surfaces. Anal. Chem. 2024, 96 (33), 13607– 13615, DOI: 10.1021/acs.analchem.4c02397
  There is no corresponding record for this reference.
83. 83
  Mapile, A. N.; LeRoy, M. A.; Fabrizio, K.; Scatena, L. F.; Brozek, C. K. The Surface of Colloidal Metal–Organic Framework Nanoparticles Revealed by Vibrational Sum Frequency Scattering Spectroscopy. ACS Nano 2024, 18 (20), 13406– 13414, DOI: 10.1021/acsnano.4c03758
  There is no corresponding record for this reference.
84. 84
  Jordan, C. J. C.; Coons, M. P.; Herbert, J. M.; Verlet, J. R. R. Spectroscopy and Dynamics of the Hydrated Electron at the Water/Air Interface. Nat. Commun. 2024, 15 (1), 182, DOI: 10.1038/s41467-023-44441-2
  There is no corresponding record for this reference.
85. 85
  Liu, C.; Qin, X.; Yu, C.; Guo, Y.; Zhang, Z. Probing the Adsorption Configuration of Methanol at a Charged Air/Aqueous Interface Using Nonlinear Spectroscopy. Phys. Chem. Chem. Phys. 2024, 26 (19), 14336– 14344, DOI: 10.1039/D3CP06317H
  There is no corresponding record for this reference.
86. 86
  Salafsky, J.; Johansson, P. K.; Abdelkader, E.; Otting, G. Ligand-Induced Conformational Changes in Protein Molecules Detected by Sum-Frequency Generation (SFG). Biophys. J. 2024, 123, 3678, DOI: 10.1016/j.bpj.2024.09.017
  There is no corresponding record for this reference.
87. 87
  Gahtori, P.; Gunwant, V.; Pandey, R. Probing the Influence of Hydrophobicity of Modified Gold Nanoparticles in Modulating the Lipid Surface Behavior Using Vibrational Sum Frequency Generation Spectroscopy. Langmuir 2024, 40 (40), 21211– 21221, DOI: 10.1021/acs.langmuir.4c02735
  There is no corresponding record for this reference.
88. 88
  Du, Q.; Superfine, R.; Freysz, E.; Shen, Y. R. Vibrational Spectroscopy of Water at the Vapor/Water Interface. Phys. Rev. Lett. 1993, 70 (15), 2313– 2316, DOI: 10.1103/PhysRevLett.70.2313
  88
  Vibrational spectroscopy of water at the vapor/water interface
  Du, Q.; Superfine, R.; Freysz, E.; Shen, Y. R.
  Physical Review Letters (1993), 70 (15), 2313-16CODEN: PRLTAO; ISSN:0031-9007.
  Using IR-visible sum-frequency generation the authors obtained the OH stretch vibrational spectra of water at the vapor/water interface. From the spectra, the authors deduce that more than 20% of the surface water mols. have one free OH projecting into the vapor. The spectrum is weakly temp. dependent from 10 to 80 °C. A monolayer of fatty alc. on water surface terminates the free OH groups and induces an icelike structure in the spectrum.
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89. 89
  Shen, Y. R.; Ostroverkhov, V. Sum-Frequency Vibrational Spectroscopy on Water Interfaces: Polar Orientation of Water Molecules at Interfaces. Chem. Rev. 2006, 106 (4), 1140– 1154, DOI: 10.1021/cr040377d
  89
  Sum-frequency vibrational spectroscopy on water interfaces: Polar orientation of water molecules at interfaces
  Shen, Yuen Ron; Ostroverkhov, Victor
  Chemical Reviews (Washington, DC, United States) (2006), 106 (4), 1140-1154CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. A review discusses the theory and exptl. arrangement of sum-frequency vibrational spectroscopy (SFVS), the SF vibrational spectra of various water interfaces, and a new SFVS technique that permits simultaneous measurements of amplitude and phase of the SF vibrational spectra for interfaces. It describes the common features of SF vibrational spectra of water interfaces, usual difficulties in interpretation of spectra, and the water interfacial structure.
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90. 90
  Sun, Q.; Guo, Y. Vibrational Sum Frequency Generation Spectroscopy of the Air/Water Interface. J. Mol. Liq. 2016, 213 (1–3), 28– 32, DOI: 10.1016/j.molliq.2015.11.004
  90
  Vibrational sum frequency generation spectroscopy of the air/water interface
  Sun, Q.; Guo, Y.
  Journal of Molecular Liquids (2016), 213 (), 28-32CODEN: JMLIDT; ISSN:0167-7322. (Elsevier B.V.)
  Vibrational sum frequency generation (SFG) spectroscopy is employed to investigate the water structure at an air/water interface. By obtaining the SFG intensity using different polarizations, it is reasonable to deconvolute the conventional SFG spectrum into five sub-bands, which are located at 3100, 3200, 3400, 3550 and 3700 cm- 1. In comparison with the Raman OH stretching bands of liq. water, we suggest that the five fitted sub-bands can be attributed to OH vibrations engaged in various local hydrogen-bonded networks, such as single donor-double acceptor (DAA), double donor-double acceptor (DDAA), single donor-single acceptor (DA), double donor-single acceptor (DDA), and free-OH vibrations, resp. Owing to the truncations of hydrogen bonds at the air/water interface, obvious structural differences between interfacial water and bulk water can be expected. For ambient water, both DA and DDAA are the primary hydrogen-bonded networks. By contrast, DA hydrogen bonding can be regarded as the primary structural motif at the air/water interface. Addnl., we suggest that the loss of DDAA hydrogen bonding at the air/water interface provides the phys. origin of surface tension, which can be applied to understand the formation of a water droplet.
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91. 91
  Ahmed, M.; Nihonyanagi, S.; Kundu, A.; Yamaguchi, S.; Tahara, T. Resolving the Controversy over Dipole versus Quadrupole Mechanism of Bend Vibration of Water in Vibrational Sum Frequency Generation Spectra. J. Phys. Chem. Lett. 2020, 11 (21), 9123– 9130, DOI: 10.1021/acs.jpclett.0c02644
  91
  Resolving the Controversy over Dipole versus Quadrupole Mechanism of Bend Vibration of Water in Vibrational Sum Frequency Generation Spectra
  Ahmed, Mohammed; Nihonyanagi, Satoshi; Kundu, Achintya; Yamaguchi, Shoichi; Tahara, Tahei
  Journal of Physical Chemistry Letters (2020), 11 (21), 9123-9130CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Recently, there has been controversy over whether the HOH bend signal of water in the vibrational sum frequency generation (VSFG) spectrum arises from the conventional dipole mechanism or the quadrupole mechanism. Here, we show that the Im χ(2) (the imaginary part of the second-order nonlinear susceptibility) spectra of the HOH bend mode of water at oppositely charged monolayer/water interfaces all exhibit pos. bands, irresp. of the difference in the sign of the charge at the interface. Furthermore, it is found that the peak frequency of the HOH bend band substantially changes depending on the chem. structure of the charged headgroup located at the interface. These results demonstrate that the VSFG signal of the HOH bend vibration is generated from interfacial water with the interfacial quadrupole mechanism that is assocd. with the large field gradient of incident lights localized in a very thin region at the interface.
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92. 92
  Du, Q.; Freysz, E.; Shen, Y. R. Surface Vibrational Spectroscopic Studies of Hydrogen Bonding and Hydrophobicity. Science 1994, 264 (5160), 826– 828, DOI: 10.1126/science.264.5160.826
  92
  Surface vibrational spectroscopic studies of hydrogen bonding and hydrophobicity
  Du, Quan; Freysz, Eric; Shen, Y. Ron
  Science (Washington, DC, United States) (1994), 264 (5160), 826-8CODEN: SCIEAS; ISSN:0036-8075.
  Surface vibrational spectroscopy by sum-frequency generation was used to study hydrophobicity at the mol. level at various interfaces: water-surfactant-coated quartz, water-hexane, and water-air. In all cases, hydrophobicity was characterized by the appearance of dangling hydroxyl bonds on 25 percent of the surface water mols. At the water-quartz interface, packing restrictions force the water surface layer to have a more ordered, ice-like structure. A partly wettable water-quartz interface was also studied.
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93. 93
  Raymond, E. A.; Tarbuck, T. L.; Richmond, G. L. Isotopic Dilution Studies of the Vapor/Water Interface as Investigated by Vibrational Sum-Frequency Spectroscopy. J. Phys. Chem. B 2002, 106 (11), 2817– 2820, DOI: 10.1021/jp013967d
  93
  Isotopic Dilution Studies of the Vapor/Water Interface as Investigated by Vibrational Sum-Frequency Spectroscopy
  Raymond, E. A.; Tarbuck, T. L.; Richmond, G. L.
  Journal of Physical Chemistry B (2002), 106 (11), 2817-2820CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
  Vibrational sum-frequency spectroscopy (VSFS) studies of dil. HOD/H2O/D2O mixts. were used to study the H-bonding interactions at the vapor/H2O interface. The decoupled nature of the HOD vibrations produces a less complex OH stretching band than the highly coupled OH stretches of H2O, creating a clearer picture of the mol. environments present in the interfacial region. From the spectra of interfacial HOD, the frequency of the uncoupled donor OH stretching mode is similar to what is obsd. in bulk HOD studies, leading to the conclusion that the H bonding environment at the surface is similar to that found in liq. H2O. This conclusion is supported by the tetrahedrally coordinated region of the vapor/water VSF spectrum, where the predominant resonant intensity is centered around 3300 cm-1, consistent with bulk Raman and IR spectra of liq. H2O.
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94. 94
  Shiratori, K.; Morita, A. Theory of Quadrupole Contributions from Interface and Bulk in Second-Order Optical Processes. Bull. Chem. Soc. Jpn. 2012, 85 (10), 1061– 1076, DOI: 10.1246/bcsj.20120167
  There is no corresponding record for this reference.
95. 95
  Yamaguchi, S.; Shiratori, K.; Morita, A.; Tahara, T. Electric Quadrupole Contribution to the Nonresonant Background of Sum Frequency Generation at Air/Liquid Interfaces. J. Chem. Phys. 2011, 134 (18), 184705 DOI: 10.1063/1.3586811
  96
  Electric quadrupole contribution to the nonresonant background of sum frequency generation at air/liquid interfaces
  Yamaguchi, Shoichi; Shiratori, Kazuya; Morita, Akihiro; Tahara, Tahei
  Journal of Chemical Physics (2011), 134 (18), 184705/1-184705/7CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
  The authors study an elec. quadrupole contribution to sum frequency generation (SFG) at air/liq. interfaces in an electronically and vibrationally nonresonant condition. Heterodyne-detected electronic sum frequency generation spectroscopy of air/liq. interfaces reveals that nonresonant χ(2) (2nd-order nonlinear susceptibility) has a neg. sign and nearly the same value for all 8 liqs. studied. This result is rationalized from the theor. expressions of χ(2) with an elec. quadrupole contribution taken into account. The nonresonant background of SFG is predominantly due to interfacial nonlinear polarization having a quadrupole contribution. Although this nonlinear polarization is localized at the interface, it depends on quadrupolar χ(2) in the bulk as well as that at the interface. It means that the sign of nonresonant χ(2) bears no relation to the up vs. down alignment of interfacial mols., because nonresonant χ(2) has a quadrupolar origin. (c) 2011 American Institute of Physics.
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96. 96
  Shiratori, K.; Yamaguchi, S.; Tahara, T.; Morita, A. Computational Analysis of the Quadrupole Contribution in the Second-Harmonic Generation Spectroscopy for the Water/Vapor Interface. J. Chem. Phys. 2013, 138 (6), 64704, DOI: 10.1063/1.4790407
  There is no corresponding record for this reference.
97. 97
  Moll, C. J.; Versluis, J.; Bakker, H. J. Direct Evidence for a Surface and Bulk Specific Response in the Sum-Frequency Generation Spectrum of the Water Bend Vibration. Phys. Rev. Lett. 2021, 127 (11), 116001 DOI: 10.1103/PhysRevLett.127.116001
  98
  Direct Evidence for a Surface and Bulk Specific Response in the Sum-Frequency Generation Spectrum of the Water Bend Vibration
  Moll, C. J.; Versluis, J.; Bakker, H. J.
  Physical Review Letters (2021), 127 (11), 116001CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
  We study the bending mode of pure water and charged aq. surfaces using heterodyne-detected vibrational sum-frequency generation spectroscopy. We observe a low (1626 cm-1) and a high (1656 cm-1) frequency component that can be unambiguously assigned to an interfacial dipole and a bulk quadrupolar response, resp. We thus demonstrate that probing the bending mode provides structural and quant. information on both the surface and the bulk.
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98. 98
  Superfine, R.; Huang, J. Y.; Shen, Y. R. Phase Measurement for Surface Infrared–Visible Sum-Frequency Generation. Opt. Lett. 1990, 15 (22), 1276– 1278, DOI: 10.1364/OL.15.001276
  There is no corresponding record for this reference.
99. 99
  Superfine, R.; Huang, J. Y.; Shen, Y. R. Experimental Determination of the Sign of Molecular Dipole Moment Derivatives: An Infrared─Visible Sum Frequency Generation Absolute Phase Measurement Study. Chem. Phys. Lett. 1990, 172 (3), 303– 306, DOI: 10.1016/0009-2614(90)85408-5
  There is no corresponding record for this reference.
100. 100
  Shen, Y. R. Phase-Sensitive Sum-Frequency Spectroscopy. Annu. Rev. Phys. Chem. 2013, 64 (1), 129– 150, DOI: 10.1146/annurev-physchem-040412-110110
  101
  Phase-sensitive sum-frequency spectroscopy
  Shen, Y. R.
  Annual Review of Physical Chemistry (2013), 64 (), 129-150CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews Inc.)
  A review. Phase-sensitive sum-frequency spectroscopy (SFS) allows the complete measurement of the complex spectra of surface nonlinear response coeffs. Similar to linear spectroscopy, the spectrum of the imaginary part of a surface response coeff. directly characterizes surface resonances without complication. This newly developed technique has greatly enhanced the capability of surface SFS and provides many new research opportunities for surface science. This article describes the exptl. schemes and underlying theory for the technique and briefly reviews works that have clearly demonstrated its power.
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101. 101
  Yamaguchi, S.; Otosu, T. Progress in Phase-Sensitive Sum Frequency Generation Spectroscopy. Phys. Chem. Chem. Phys. 2021, 23 (34), 18253– 18267, DOI: 10.1039/D1CP01994E
  102
  Progress in phase-sensitive sum frequency generation spectroscopy
  Yamaguchi, Shoichi; Otosu, Takuhiro
  Physical Chemistry Chemical Physics (2021), 23 (34), 18253-18267CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  A review. Sum frequency generation (SFG) spectroscopy is a unique and powerful tool for investigating surfaces and interfaces at the mol. level. Phase-sensitive SFG (PS-SFG) is an upgraded technique that can overcome the inherent drawbacks of conventional SFG. Here we review several methods of PS-SFG developed and reported in 1990-2020. We introduce how and by which group each PS-SFG method was designed and built in terms of interferometer implementation for optical heterodyne detection, with one exception of a recent numerical method that does not rely on interferometry. We also discuss how PS-SFG solved some typical problems for aq. interfaces that were once left open by conventional SFG. These problems and their solns. are good examples to demonstrate why PS-SFG is essential. In addn., we briefly note a few terminol. issues related with PS-SFG to avoid confusion.
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102. 102
  Xu, X.; Shen, Y. R.; Tian, C. Phase-Sensitive Sum Frequency Vibrational Spectroscopic Study of Air/Water Interfaces: H2O, D2O, and Diluted Isotopic Mixtures. J. Chem. Phys. 2019, 150 (14), 144701 DOI: 10.1063/1.5081135
  103
  Phase-sensitive sum frequency vibrational spectroscopic study of air/water interfaces: H2O, D2O, and diluted isotopic mixtures
  Xu, Xiaofan; Shen, Y. Ron; Tian, Chuanshan
  Journal of Chemical Physics (2019), 150 (14), 144701/1-144701/5CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
  Using phase-sensitive sum-frequency vibrational spectroscopy with a carefully chosen phase ref., we revisited the vibrational spectra of vapor/water interfaces of neat H2O and D2O as well as HDO in dild. isotopic mixts. Using z-cut quartz as the phase ref., with proper frequency scaling, the gross features of the OH and OD stretching spectra from H2O and D2O and from HDO in two isotopic mixts. look similar and agree with those reported earlier, but differences are also apparent. In particular, a weak pos. band at low frequencies, which has been asserted by mol. dynamic simulations but not detectable in the exptl. OH spectrum with pure H2O, is now visible in the OD spectrum. The differences must arise from the change of intermol. interaction of water mols. with their surrounding mols. upon exchange of the isotopes. (c) 2019 American Institute of Physics.
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103. 103
  Sathyanarayana, D. N. Vibrational Spectroscopy: Theory and Applications; New Age International (P) Ltd.: New Delhi, 2004.
  There is no corresponding record for this reference.
104. 104
  Lagunov, O.; Drenchev, N.; Chakarova, K.; Panayotov, D.; Hadjiivanov, K. Isotopic Labelling in Vibrational Spectroscopy: A Technique to Decipher the Structure of Surface Species. Top. Catal. 2017, 60 (19), 1486– 1495, DOI: 10.1007/s11244-017-0833-x
  There is no corresponding record for this reference.
105. 105
  Sun, S.; Tian, C.; Shen, Y. R. Surface Sum-Frequency Vibrational Spectroscopy of Nonpolar Media. Proc. Natl. Acad. Sci. U. S. A. 2015, 112 (19), 5883– 5887, DOI: 10.1073/pnas.1505438112
  106
  Surface sum-frequency vibrational spectroscopy of nonpolar media
  Sun, Shumei; Tian, Chuanshan; Shen, Y. Ron
  Proceedings of the National Academy of Sciences of the United States of America (2015), 112 (19), 5883-5887CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
  Sum-frequency generation spectroscopy is surface specific only if the bulk contribution to the signal is negligible. Negligible bulk contribution is, however, not necessarily true, even for media with inversion symmetry. The inevitable challenge is to find the surface spectrum in the presence of bulk contribution, part of which was believed to be inseparable from the surface contribution. Here, for nonpolar media, it is possible to sep. deduce surface and bulk spectra from combined phase-sensitive sum-frequency vibrational spectroscopic measurements in reflection and transmission. The study of benzene interfaces is presented as an example.
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106. 106
  Hore, D. K.; Tyrode, E. Probing Charged Aqueous Interfaces Near Critical Angles: Effect of Varying Coherence Length. J. Phys. Chem. C 2019, 123 (27), 16911– 16920, DOI: 10.1021/acs.jpcc.9b05256
  107
  Probing Charged Aqueous Interfaces Near Critical Angles: Effect of Varying Coherence Length
  Hore, Dennis K.; Tyrode, Eric
  Journal of Physical Chemistry C (2019), 123 (27), 16911-16920CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Angle-resolved vibrational sum frequency generation expts. have been used to study the silica-water interface as a function of ionic strength. Well below the crit. angle, the sum frequency intensity increases up to 10-4 M NaCl and then drops. However, near the crit. angle, a plateau may be obsd. up to 10-4 M. We first demonstrate that this is a result of the interaction of a long Debye length at low ionic strength with a long coherence length near the crit. angles. In order to account for the behavior at the lowest concns. where surface potentials are typically large, it is necessary to consider an electrostatic potential that extends into the bulk aq. phase beyond the Debye-Huckel approxn. Because the extent of second- and third-order contributions to the nonlinear polarization can vary with ionic strength, but not with the angle of incidence, we perform a global fit to the exptl. data using our proposed model to ext. the relative magnitude of the two susceptibilities. The ionic strength dependence of this ratio points to the crit. nature of the silanol deprotonation and the development of surface charge and illustrates how surface water mols. respond. These results highlight the importance of varying the coherence length in order to probe the water structure at charged interfaces.
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107. 107
  Cai, C.; Azam, M. S.; Hore, D. K. Determining the Surface Potential of Charged Aqueous Interfaces Using Nonlinear Optical Methods. J. Phys. Chem. C 2021, 125 (45), 25307– 25315, DOI: 10.1021/acs.jpcc.1c07761
  108
  Determining the Surface Potential of Charged Aqueous Interfaces Using Nonlinear Optical Methods
  Cai, Canyu; Azam, Md. Shafiul; Hore, Dennis K.
  Journal of Physical Chemistry C (2021), 125 (45), 25307-25315CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Second-order nonlinear optical techniques have recently been established as sensitive probes of charged interfaces through the nonlinear susceptibility of water and provide an attractive route to elucidate the surface potential. We discuss methods that have been proposed using electronic second-harmonic generation and vibrational sum-frequency generation. A detailed comparison is provided, using a unified notation and including a discussion of the assumptions that are either convenient or necessary in order to arrive at the surface potential. We then illustrate that, when using the full soln. of the Poisson-Boltzmann equation that is applicable to a wide range of surface potentials, several benefits may be realized. The first is that, when using off-resonance phase measurements, the surface potential can be detd. from the phase alone, without any addnl. information or calibrated intensity schemes. Next, we illustrate a scheme for surface potential measurement using intensity-only data off-resonance by changing the angle of incidence. Finally, we discuss the possibility of measurements that target the vibrational resonance with water and illustrate how the surface potential can be isolated from such data.
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108. 108
  Yu, C.-C.; Seki, T.; Wang, Y.; Bonn, M.; Nagata, Y. Polarization-Dependent Sum-Frequency Generation Spectroscopy for \AA{}ngstrom-Scale Depth Profiling of Molecules at Interfaces. Phys. Rev. Lett. 2022, 128 (22), 226001 DOI: 10.1103/PhysRevLett.128.226001
  There is no corresponding record for this reference.
109. 109
  Yu, C.-C.; Seki, T.; Chiang, K.-Y.; Tang, F.; Sun, S.; Bonn, M.; Nagata, Y. Polarization-Dependent Heterodyne-Detected Sum-Frequency Generation Spectroscopy as a Tool to Explore Surface Molecular Orientation and Ångström-Scale Depth Profiling. J. Phys. Chem. B 2022, 126 (33), 6113– 6124, DOI: 10.1021/acs.jpcb.2c02178
  There is no corresponding record for this reference.
110. 110
  Tan, J.; Wang, M.; Zhang, J.; Ye, S. Determination of the Thickness of Interfacial Water by Time-Resolved Sum-Frequency Generation Vibrational Spectroscopy. Langmuir 2023, 39 (50), 18573– 18580, DOI: 10.1021/acs.langmuir.3c02906
  There is no corresponding record for this reference.
111. 111
  Sung, W.; Inoue, K.; Nihonyanagi, S.; Tahara, T. Unified Picture of Vibrational Relaxation of OH Stretch at the Air/Water Interface. Nat. Commun. 2024, 15 (1), 1258, DOI: 10.1038/s41467-024-45388-8
  There is no corresponding record for this reference.
Supporting Information
Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.4c06650.
- Materials and methods used in this work, discussion on the approximations in the SFG response from charged interfaces, effect due to modulating the coherence length (PDF)
- jp4c06650_si_001.pdf (225.92 kb)
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Sum-Frequency Generation Spectroscopy of Aqueous Interfaces: The Role of Depth and Its Impact on Spectral InterpretationClick to copy article linkArticle link copied!

The Journal of Physical Chemistry C

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1. Introduction

2. The Importance of Depth in SFG Measurements

2.1. The 2D “Picture” of the Interface

Figure 1

2.2. The Experimentally Accessible Quantity in SFG

2.3. Obtaining Absolute Units

Figure 2

2.4. Charged Interfaces

Figure 3

2.5. Entanglement with Experimental Parameters

3. Complications for Structural Interpretations and Molecular Recognition

3.1. Depth Related Line-Shape Distortions

3.2. Interference between Multiple Contributions

Figure 4

3.3. Quadrupolar Contributions

Figure 5

4. Recent Developments in Probing Aqueous Interfaces

Figure 6

4.1. Transmission/Reflection SFG

4.2. Variable-Angle SFG

4.3. Frequency (Momentum)-Dependent SFG

4.4. Combining Sum- and Difference Frequency Generation (SFG/DFG)

Figure 7

4.5. Summary of Coherence Length Modulation Techniques

Figure 8

4.6. Other Methods

5. Future Directions

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Alexander P. Fellows

Álvaro Díaz Duque

Vasileios Balos

Louis Lehmann

Roland R. Netz

Martin Wolf

Martin Thämer

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Alexander P. Fellows

Álvaro Díaz Duque

Vasileios Balos

Louis Lehmann

Roland R. Netz

Martin Wolf

Martin Thämer

References

Supporting Information

Supporting Information

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Sum-Frequency Generation Spectroscopy of Aqueous Interfaces: The Role of Depth and Its Impact on Spectral Interpretation
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