On the Excitation Mechanism of Vibrational Modes of C₂H₄ Chemisorbed on Pt(111) and Ni(111) by EELS

Abstract

The angular dependence of the vibrational loss intensities of C₂H₄ chemisorbed on Pt(111) and Ni(111) have been studied by non-specular EELS measurements. The analysis of the angular profiles permits the separation of the contributions from dipole and impact scattering. For Pt(111) all the angular range is dominated by impact scattering except for the C-Pt stretch mode which is dipole. For Ni(111) dipole scattering is important and dominates the intensity contribution in the specular direction. In both cases impact scattering is found to increase at low energies (2 eV). The values of the differential cross section for impact scattering are calculated and compared with those observed by electron impact on free ethylene. The implications of these results for the interpretation of the spectra are outlined. We present also the vibrational spectrum of the room temperature phase of ethylene chemisorbed on Pt(111) recorded in and out of specular, to illustrate the importance of the excitation mechanism for chemical analysis. The analysis of both spectra indicates that two different species are present on the surface each one being excited by a different scattering mechanism.