Electron-driven dynamics at the gas/solid interface: dissociation, desorption and reaction of adsorbed molecules

Abstract

This paper considers dynamical processes which can be initiated by low-energy (1–50 eV) electrons in adsorbed molecular layers. We have investigated the production of negative ions by electron-stimulated desorption from well ordered monolayer and multilayer films of O₂ on graphite. Resonances are observed in the yield of both O^– and O^–₃ ions, and are attributed to the process of dissociative electron attachment. In the monolayer regime, the 8 eV resonance which dominates the O^– yield at higher coverages is found to be suppressed, and dipolar dissociation dominates. This suppression is attributed to the image potential, which attracts low-energy ions back to the surface. The angular distribution of O^– ions desorbed from the monolayer δ and ζ phases we found to be almost independent of the initial molecular orientation on the surface. Classical trajectory calculations indicate that the molecule becomes rotationally excited prior to dissociation, causing the initial orientational order to be lost. This marks a difference between the dissociation and desorption dynamics of physisorbed and chemisorbed molecules, where the angular distribution of desorbed fragments is generally taken to reflect the molecular orientation on the surface.