Introduction

The surface vibrations of adsorbates on crystal surfaces may be studied by Infrared Reflection—Absorption Spectroscopy (IRAS), Raman spectroscopy, High Resolution Electron Energy Loss Spectroscopy (HREELS) or molecular beam scattering. The last named is the subject of a separate chapter and will not be discussed further here. In all of these techniques, with the exception of molecular beam scattering, the inelastic process results from the same physical entity, the vibrating surface dipole, and to a certain extent similar selection rules apply for IRAS and HREELS; the selection rules for Raman activity complement those of IRAS.

One would expect that these techniques would yield much the same sort of structural information about adsorbed species on metal surfaces. This is generally true, though direct comparisons are not always possible since IRAS and Raman spectroscopy may be used at quite high pressures, akin to those encountered in catalytic systems, while HREELS may not. Balanced against this apparent disadvantage for HREELS is the fact that it can readily scan 1 eV, the whole infrared range, in one experiment. Unfortunately, until recently, the resolution obtained for HREELS did not exceed ∼ 5 meV; one must contrast this with the resolution attainable with IRAS, which is typically at least 0.05 meV.