Benzene adsorbed on silver: An electron energy loss and surface-enhanced Raman study

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Abstract
Vibrational electron energy loss(EELS) and surface‐enhanced Raman (SERS)spectra of benzene on evaporated silver display marked coverage dependence for ‘‘cold‐deposited’’ surfaces prepared at or above 150 K. In contrast, silverdeposition at 60 K leads to less complex behavior. On ‘‘warm’’ prepared surfaces, 150–300 K, there exists a low coverage spectrum and a distinctly different high coverage (still submonolayer) spectrum. On cold prepared surfaces, 60 K, only one type of spectrum appears and it closely resembles the high coverage spectrum seen on a warm prepared surface. The dependence on silver preparation temperature, evidence that different sites lead to different spectra, and the expectation that benzene forms substantial π bonds only, implies that this is not a compressional reorientation effect like that observed previously for pyridine on Ag(111). It appears that cold prepared surfaces have very rough features giving rise to only one type of spectrum, while warm prepared surfaces have smooth and rough areas resulting in the low and high coverage spectra, respectively. The low and high coverage spectra may be an indication that the two types of site support unique orientations. Since however, orientation alone does not account for all observed features it may be that the rough silversurface provides a degeneracy lifting mechanism, possibly a field gradient effect like that proposed in relation to SERS.
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