HREELS surface hydrogen vibrations

The hydrogenation is catalysed by nickel and other transition metals. Vibrational spectroscopy (infrared, HREELS, INS) has been applied to determining the orientation and binding of benzene on the catalyst surface. We suimnarise INS studies of benzene, adsorbed benzene and model complexes. The intensities and frequencies of the vibrational modes are computed by the Wilson GF method. The benzene molecule... 

Figure 4.3. Vibrational spectra of hydrogen and deuterium adsorbed on the W(IOO) surface obtained by HREELS. The curves correspond to the spectrum obtained at different coverages [89] d.

Here the adsorbed atomic hydrogen is assumed to be localized on a specific site leading to out of N sites being occupied. In this case the three degrees of freedom of the adsorbed hydrogen atom are reduced to three fmstrated vibrations which, in principle, can be measured by HREELS [191] or alternatively estimated by DET. Anyway this is typically the case for the two dipole forbidden fmstrated vibrations parallel to the surface since they are very weak in a HREELS experiment. 

Figures 1 Specular and off-specular HREELS spectra of H adsorbed on Pd(100). The perpendicular mode is at 63 meV. The parallel mode is at 76 meV. Reprinted from Surface Science, 178, M.E. Kordesch, Surface resonances in vibrational spectroscopy of hydrogen on transition metal surfaces Pd(IOO) and Pd(111), 578-588,1986, with permission from Elsevier Science.

Atomic hydrogen is small and adsorbs near to the surface. H modes can directly couple to surface states and resonances, such as image potential states, that may overlap in the near-surface region. An empirical verification of this phenomenon, observed on several metals (Pd, Pt, Rh, Ru) is the enhancement of surface resonances by adsorbed H, and the enhancement of H vibrational modes at primary energies which correspond to the population of the surface resonance with electrons from the incident HREELS beam. In Figure 17, the reflectivities of the Pd(lll) and (100) surface with and without adsorbed hydrogen are shown. The relative intensities of the H frustrated translation and rotation (perpendicular and parallel) modes are shown in Figure 18. 

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