Vibrational spectra of C 6H 6 and C 6H 5D adsorbed on Pd(111) : A comparative study of electron energy loss spectra

Abstract

Vibrational spectra of C 6H 6 and C 6H 5D adsorbed on Pd(111) have been studied by electron energy loss spectroscopy. The loss spectrum of the C 6H 5D adsorbate is analysed via comparison with the corresponding IR spectra of free C 6H 5D. This analysis facilitates the interpretation of the previously studied spectrum of C 6H 6 chemisorbed on Pd. A comparison between loss spectra of C 6H 6 and C 6H 5D adsorbates supports the interpretation that the two strong losses of the C 6H 6 adsorbate are out-of-plane hydrogen bending modes. By correlating these losses to v 11(A 2u) and v 10(E 1g) modes of free C 6H 6, a bridge binding geometry is implied for the aromatic ring lying flat on the metal surface. Evidence is provided for a reinterpretation of some weak losses, which are correlated with IR-active in-plane modes. For their appearance an alternative mechanism instead of mode mixing is proposed which involves oscillating motions of the aromatic ring against the metal surface.