CO Adsorption on Reconstructed Ir(100) Surfaces from UHV to mbar Pressure: A LEED, TPD, and PM-IRAS Study

Clean and stable surface modifications of an iridium (100) single crystal, i.e., the (1 × 1) phase, the (5 × 1) reconstruction, and the oxygen-terminated (2 × 1)-O surface, were prepared and characterized by low energy electron diffraction (LEED), temperature-programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS) and polarization modulation IRAS (PM-IRAS). The adsorption of CO in UHV and at elevated (mbar) pressure/temperature was followed both ex situ and in situ on all three surface modifications, with a focus on mbar pressures of CO. The Ir(1 × 1) surface exhibited c(4 × 2)/c(2 × 2) and c(6 × 2) CO structures under low pressure conditions, and remained stable up to 100 mbar and 700 K. For the (2 × 1)-O reconstruction CO adsorption induced a structural change from (2 × 1)-O to (1 × 1), as confirmed by LEED, TPD, and IR. For Ir (2 × 1)-O TPD indicated that CO reacted with surface oxygen forming CO2. The (5 × 1) reconstruction featured a reversible and dynamic behavior upon CO adsorption, with a local lifting of the reconstruction to (1 × 1). After CO desorption, the (5 × 1) structure was restored. All three reconstructions exhibited CO adsorption with on-top geometry, as evidenced by IR. With increasing CO exposure the resonances shifted to higher wavenumber, due to adsorbate–adsorbate and adsorbate–substrate interactions. The largest wavenumber shift (from 2057 to 2100 cm–1) was observed for Ir(5 × 1) upon CO dosing from 1 L to 100 mbar.


XPS Measurements
XPS was performed with a non-monochromatic X-ray source (Specs XR 50, with AlKα and MgKα anode) combined with a Specs EA 150 PHOIBOS hemispherical analyzer. The spectra were recorded with AlKα at an angle of 60° between surface normal and analyzer. For all examined surfaces, their cleanliness was confirmed by XPS. In particular, C1s spectra were acquired to exclude the presence of carbonaceous deposits before the adsorption experiments.
Furthermore, XPS spectra taken after adsorption experiments confirmed the absence of CO dissociation, i.e. only molecular CO was present during the experiments.
For evaluation of the C1s signal an overlap of the AlKα satellite feature of the Ir4d signal with the C1s signal has to be considered. Figure S1 and S2 show the Ir4d/C1s signal of the pristine surface and of the surface after CO adsorption/desorption for (2x1)-O and (5x1). As can be seen S2 in the figures no carbon formation during the experiments was observed, confirming that CO only adsorbs molecularly on the surface (no CO dissociation). Figure S1. Ir4d and C1s XPS of the (2x1)-O reconstruction in its pristine state and after CO adsorption/desorption experiments. Note that the low energy feature of the Ir4d peak and the C1s signal are overlapping. Figure S2. Ir4d and C1s XPS of the (5x1) reconstruction in its pristine state and after CO adsorption/desorption experiments. Note that the low energy feature of the Ir4d peak and the C1s signal are overlapping. Figure S3 shows O1s XPS from the (2x1)-O surface in its pristine state and after CO adsorption followed by TPD (at the end of the TPD series). It can be clearly seen that the amount of oxygen that is bound to the surface is strongly reduced in the course of the TPD experiment (CO is reacting with the surface oxygen to CO 2 ). Figure S3. O1s XPS peak of the pristine 2x1-O reconstruction and of the surface after CO adsorption and TPD.

Discussion and assignment of LEED structures
The LEED patterns in Figures 2 and 3, observed for CO saturation coverage (as well as during the in-situ LEED series at 300 K and 250 K) can either be attributed to a c(2x2) structure with spot splitting or to a c(4x2) structure with systematic spot extinctions. Both structures could be present, as discussed in the following.
The c(2x2) spot splitting may be caused by antiphase domain boundaries (see figure S4a). At the domain boundaries the local coverage may be different from the overall coverage (i.e. it can be higher than ½ ML as additional CO molecules are inserted). In our case the splitting of the former (1/2, 1/2) spot is not small, but a substantial fraction of the (shortest) reciprocal lattice vector; it is ¼ of the reciprocal unit cell length. Accordingly, in real space this relates to a periodic S5 occurrence of domain boundaries every 4 atoms of the underlying substrate. Since a minimum of one CO molecule would be inserted into such a mini-domain (or into the domain boundary, respectively), the coverage of one unit cell would increase by 1 CO molecule per 4 substrate atoms (that is 0.25 ML). Consequently, relative to the c(2x2) structure, this would lead to an overall coverage of 0.75 ML. However, due to the high density of the domain boundaries (every 4 th substrate atom), the distinction between the interior of the domain and the domain boundary may become ambiguous.
With respect to a "true" c(4x2) structure ( Figure S4b), there is no qualitative difference to a c(2x2) structure with spot splitting (and the periodic occurrence of antiphase domain boundaries).
Nevertheless, for a real c(4x2) overlayer structure some diffraction spots are missing. This may be due to intensity reasons but one could also argue that if there are more than one CO molecule per unit cell (here 3 CO molecules per primitive unit cell), systematic spot extinctions may occur, due to destructive interference within one unit cell ("structure factor" of diffraction). In this case only a subset of the expected LEED spots would be present at all electron energies, in agreement with experiment.
Currently, it is a matter of debate whether the overlayer structure is termed c(4x2) with systematic spot extinctions or c(2x2) with spot splitting. For a final answer more detailed LEED studies and simulations, or even STM studies, would be required. substrate (blue) and the red arrows to the CO overlayer (red).