Contrasting Dynamics in Isoelectronic Anions Formed by Electron Attachment

Cyanogen NCCN and cyanoacetylene HCCCN are isoelectronic molecules, and as such, they have many similar properties. We focus on the bond cleavage in these induced by the dissociative electron attachment. In both molecules, resonant electron attachment produces CN– with very similar energy dependence. We investigate the very different dissociation dynamics, in each of the two molecules, revealed by velocity map imaging of this common fragment. Different dynamics are manifested both in the excess energy partitioning and in the angular distributions of fragments. Based on the comparison with electron energy loss spectra, which provide information about possible parent states of the resonances (both optically allowed and forbidden excited states of the neutral target), we ascribe the observed effect to the distortion of the nuclear frame during the formation of core-excited resonance in cyanoacetylene. The proposed mechanism also explains a puzzling difference in the magnitude of the CN– cross section in the two molecules which has been so far unexplained.

DEA to HC 3 N Here we present additional experimental data on the DEA to cyanoacetylene.Figure 1 shows the time-of-flight mass spectrum of anionic fragments which demonstrates that the individual ions are resolved and that the central slice images do not contain significant contribution of ions with neighbouring masses.
Figure 2 shows the images of the four fragments recorded at 5.3 eV incident electron energy.The general character of the image (central blob which is typical for a statistical distribution of kinetic energies) is the same for all the fragments and does not change at different incident energies.Figure 3 shows the kinetic energy distributions of fragments obtained from the images.The distributions change only very weakly with the electron energy.

DEA to NCCN: analysis of angular distributions
A theory relating the angular distribution of the anionic fragments produced due to dissociative electron attachment (DEA) and the symmetry of the resonance state was given by O'Malley and Taylor. 1 The theory was developed for diatomic molecules and considering only a single resonance state is involved.The coupling is due to pure electronic matrix later generalised this theory to treat polyatomic molecules.The angular distribution of the anionic fragment in the laboratory frame with respect to the incident electron beam direction ϑ can be expressed as I ϵ (ϑ) for each resonant state ϵ, integrating of the azimuthal angle φ where ϑ and φ are the polar angles of the electron beam, X ϵ l,m (ϑ, φ) are the basis functions for the irreducible representations of the point group G of the molecule in the dissociation frame.
The expansion coefficients a l,m are real numbers, whereas the δ l are the phase differences between partial waves with respect to the lowest one involved.
Based on equation ( 1) the expression in equation ( 2) was obtained and used to fit the experimentally obtained angular distribution data.
In the above expression, the quantity µ corresponds to the change in orbital angular momentum between the initial and the final molecular state.It can be defined as, µ =

Time-dependent DFT study of NCCN and HCCCN
The TD-DFT calculations employed the B3LYP functional and the cc-pVTZ basis set 3 as implemented in Gaussian 16. 4 The neutral and anion excited states will be referred with respect to the following ground state configurations: Excited neutral states Vertical energies of the triplets are calculated as excitations of the tripled ground state.
For the electron-molecule collision purposes it is convenient to present them relative to the   singlet neutral ground state as shown in Tables 2 and 4. Note that all the energies listed here are relative to the neutral ground state in the equilibrium geometry.Therefore, their values will be generally higher when compared to the calculations of Fischer and Ross 5 that list the adiabatic (geometry-relaxed) energies of the excited states.

Negative ion states of NCCN
Energies of the negative ion states presented here are relative to the neutral ground state in the equilibrium geometry.Values are summarized in Tab. 5.

Figure 1 :
Figure 1: Mass spectrum of DEA fragments from HC 3 N at 5.3 eV electron energy.

Figure 2 :Figure 3 :
Figure 2: VMI images of four anionic fragments produced due to DEA to HC 3 N molecule at 5.3 eV incident electron energy.Each images are of 5 ns width around the centre of the 'Newton sphere' and the small red arrows indicate the direction of the incident electron beam.
where Λ i and Λ f are the projection of the electronic axial orbital momentum along the molecular axis for the initial and final molecular states, respectively.The summation index l ≥ |µ| can further be restricted between only even or odd values depending on whether the initial and final state have the same parity or not.The a l 's are the expansion coefficients of each partial wave of the spherical harmonics Y l,µ .To compute the hypothetical CN − /NCCN velocity map image (shown in Fig.3of the main text), the involvement of a Σ g resonant state is considered.As both the initial and final states are of Σ g symmetries, the change in orbital angular momentum, µ = 0 and also due to the same parity of both states, the values of l are further restricted only to even numbers.In the present case, the first three partial waves with l=0, 2 and 4 which can lead a Σ g to Σ g transition are considered.The values of different fitting parameters obtained for the best fit of the angular distribution which was further used to compute the hypothetical VMI image are as follows: a 0 =3.306; a 2 =0.5901; a 4 =0.1399;d 2 =4.159; d 4 =3.657.As already defined, d 2 and d 4 are the phase differences between the s-wave (with l = 0) and d-wave (with l = 2) and in-between s-wave and g-wave (with l = 4), respectively and a l 's are the expansion coefficient of each partial waves, with the given l values.

Table 1 :
Vertical energies of the excited singlet states of NCCN.

Table 2 :
Vertical energies of the excited triplet states of NCCN relative to the singlet ground state.

Table 3 :
Vertical energies of the excited singlet states of HCCCN.

Table 4 :
Vertical energies of the excited triplet states of HCCCN relative to the singlet ground state.