A COMPARATIVE-STUDY OF THE ELECTRON-ENERGY LOSS SPECTRUM AND THE SURFACE-ENHANCE RAMAN-SPECTRUM OF BENZENE ADSORBED ON SILVER

EELS spectra of benzene adsorbed on cold-deposited silver films prepared at different temperatures, then cooled to 60 K before adsorbate dosing, are found to contain several normally ir-forbidden bands whose relative intensities are observed to be dose dependent, especially for surfaces prepared above 200 K. In contrast, EELS spectra of benzene adsorbed on 60 K surfaces do not change with dose and resembled the high-coverage spectra obtained with surfaces prepared at the higher temperatures. These observations are rationalized in terms of a model that assumes benzene can adsorb on silver at two types of sites, binding somewhat more strongly to one type than to the other. Dipole-forbidden modes originate from the more weakly bound adsorbate and draw their intensity, at least partly, from resonant scattering by a resonance of a transient benzene anion. These resonances appear to be suppressed in the more strongly bound benzene. EELS spectra of adsorbed pyridine, triazine, and ethylene show no indications of such resonant contributions. Impact scattering also seems to contribute unusually greatly to the intensity of several of the observed EELS bands, especially for surfaces prepared at very low temperatures, because of their roughness, causing the specular nature of dipolar scattering to be diminished and with it the special intensity enjoyed by dipole-allowed bands. SERS spectra of benzene adsorbed on cold-deposited silver and the progress of the relative intensities of bands with dose cannot be wholly reconciled with what is observed with EELS. For, example, the existence of two types of binding sites for benzene is not implied as strikingly by the SERS results. This behavior implies that adsorbed benzene molecules do not contribute equally to EELS and SERS. In particular the more strongly bound benzene molecules appear to contribute disproportionately greatly to the SERS signal to a degree that can not be ascribed entirely to the differing selection rules operating in the two surface spectroscopies.