Electron Scattering by Adsorbed Molecules: Dynamics of the Resonant Vibrational Excitation

Abstract

The formation of a transient negative ion AB⁻, a resonance, in the course of an electron-molecule collision is well known to be the origin of an important vibrational excitation process (Schulz, 1973). Indeed, the lengthening of the collisional time due to the trapping of the electron in the resonant state increases the energy transfer between the electronic and nuclear movements. This argument also holds for molecules in an environment such as a solid surface and indeed resonant electron scattering is the origin of a strong vibrational excitation in adsorbed molecules. The first experimental observations of this process goes back to 1981 (Sanche et Michaud 1981, Demuth et al 1981): vibrational excitation of molecules adsorbed on a metal surface, studied by high resolution electron energy loss spectroscopy (HREELS) revealed the presence of resonance structures. These experiments were performed in physisorbed molecules weakly perturbed by the surface and the resonances observed in adsorbed molecules could be associated with well known resonances in the isolated molecules. However, these resonances appear to be modified by the metal surface environment: their energy is lower and their lifetime is shorter than that of their correspondent in the isolated molecule. The study by HREELS of the resonant vibrational excitation process has developed since these pioneering works and basically it went along two directions: the use of resonant scattering as an analytical tool and the study of the vibrational excitation and its consequences. These studies have been reviewed in recent publications (Sanche 1990, Palmer and Rous 1992, Palmer 1992). The first direction is to use resonant electron scattering as an analytical tool for studying the geometry of the adsorbate.