An electron energy loss study of the surface formate species chemisorbed on Ni(110): Dipole, impact, and resonance scattering for adsorbate covered surfaces

Abstract

A comprehensive energy and angular dependent high resolution electron energy loss study of the surface formate species (HCOO), adsorbed on Ni(110) at room temperature, has been carried out. By varying both the primary electron beam energy Ei and the scattering geometry, dipole, impact, and resonance scattering processes have been identified and exploited, to gain information regarding the different vibrational excitations of the HCOO/Ni(110) system. Application of the appropriate dipole and impact scattering selection rules, yields information concerning the HCOO orientation at the surface. It is found that the HCOO bonds to the surface, via the two oxygen atoms, with the C2 axis normal to the surface and with the plane of the HCOO aligned along the close-packed 〈110〉 rows. A dipole active Ni phonon, at 29 meV, observed in specular scattering geometry, is assigned to an S̄ phonon of the clean surface Brillouin zone and clearly indicates that the HCOO is adsorbed with the C–H bond projecting onto a short-bridge site, with the two oxygen atoms occupying effectively on-top sites. Finally, enhancement of the symmetric O–C–O stretch is attributed to a resonant scattering process, centered at an incident electron beam energy, Ei=14 eV, resulting in the formation of a temporary negative ion. On the basis of the energy and angular dependence of the enhancement, the resonant state is related to the so-called σ-shape resonances recently observed in near-edge x-ray absorption spectroscopy studies of HCOO adsorbed on copper surfaces. Overtone and combination bands, some of which are also enhanced by the resonant excitation, are also assigned.