METAL-SURFACE RAMAN-SPECTROSCOPY - THEORY

Studies of the Raman and i.r. spectra of several molecules adsorbed to metal surfaces indicate that special selection rules govern these spectra. In the case of i.r. reflection adsorption spectroscopy (IRS) as in the related techniques of electron energy loss spectroscopy (ELS) and inelastic electron tunneling spectroscopy (IETS), only modes which develop dipole moment perpendicular to the metal surface can be excited. We have shown that an image field model which has been proposed to account for these observations is equivalent to the presence of a new symmetry operation which consists of simultaneous reflection in the plane of the metal surface plus charge conjugation. As a result, instead of the point group of the molecule, the symmetry group which should be used to analyze the motions of a molecule adsorbed to a metal surface consists of the direct product group of the molecular point group and another which is isomorphous with C2. Furthermore, the activity representations of the system, molecule + image, for both i.r. and Raman spectra, are those which are compatible with the totally symmetric representation of C2. Both i.r. and Raman spectra of oriented metal adsorbates can therefore be used to probe molecular orientation. Depolarization ratios for specific orientations and scattering geometries have been derived and used to analyze the results of several experimental examples. The adsorbed molecule whose Raman spectrum has been most thoroughly studied is pyridine on silver. Using secondary ion mass spectroscopy, we have been able to show that the silver surfaces are atomically quite clean. The case of pyridine adsorbed on Ag is also of interest because of the relative intensity of some of its Raman lines. By comparison with the same lines in liquid pyridine, the intensity is apparently enhanced by a factor of 104. Several theories of this enhancement are analyzed, including the possibility that the enhancement is a demonstration of the resonance Raman effect in which the surface plasmons mix with molecular electronic states so as to form a continuum of intermediate states for the scattering. Each theory implies a particular dependence of the Raman intensity on the frequency of the exciting radiation. Resonance interaction with the surface plasmons gives the closest agreement with experimental results. © 1979.