Atomic adsorption vibration spectrum

Figure Bl.25.12 illustrates the two scattering modes for a hypothetical adsorption system consisting of an atom on a metal [3]. The stretch vibration of the atom perpendicular to the surface is accompanied by a change m dipole moment the bending mode parallel to the surface is not. As explained above, the EELS spectrum of electrons scattered in the specular direction detects only the dipole-active vibration. The more isotropically scattered electrons, however, undergo impact scattering and excite both vibrational modes. Note that the comparison of EELS spectra recorded in specular and off-specular direction yields infomiation about the orientation of an adsorbed molecule.

Figure 5 HREEL spectra recorded after small O2 doses at E = 0.39 eV on flat Ag(l 0 0) (bottom spectrum), Ag(41 0) (middle) and Ag(2 1 0) (top) at T = 105 K. O2 is dosed at normal incidence for Ag(l 0 0) and close to the normal to the step heights for the stepped surfaces. The losses in the 30-40 meV region are due to adatom surface vibrations, those at 80-84 meV to the internal stretch mode of the O2 admolecules. It is evident that only molecular adsorption takes place on flat Ag(l 00), while adatoms and admolecules coexist on Ag(41 0) and the final adsorption state is purely dissociative for Ag(2 1 0). The residual intensity at 84meV in the upper spectrum is most probably due to imperfections of the (21 0) staircase leading to larger terraces. We remind that the oxygen dose is expressed in ML of surface atoms, which are therefore referred to the corresponding face density.

In light of the open questions related to CO adsorption/dissociation on Rh(l 1 1), Pery et al 314) carried out a systematic SFG/AES study of CO on Rh(l 1 1), at pressures from 10 to 1000 mbar and temperatures from 300 to 800 K. Figures 38a and b show a series of SFG spectra recorded at 300 K and a comparison of spectra at 10 mbar before and after the atmospheric pressure gas exposure. All spectra are dominated by a single vibrational peak at 2053-2075 cm typical of CO terminally bonded to a single Rh atom, with a small peak at about 1900 cm characterizing CO on threefold hollow sites (see, e.g., the 500-mbar spectrum). The intensity difference between the two peaks again points to the lower sensitivity of... 

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