SELECTION-RULES IN RESONANCE ELECTRON-SCATTERING FROM ADSORBED MOLECULES

We demonstrate the existence of selection rules for resonance electron scattering from adsorbed molecules observed in high-resolution electron-energy-loss spectroscopy which give rise to nodes in the angular distribution of inelastically scattered electrons. These extinctions, which are expected to occur for isolated oriented molecules, survive adsorption even when the presence of eleastic multiple scattering strongly perturbs the angular emission profiles. These nodes occur at fixed angles to the surface normal and are characteristic of both the resonance symmetry and the adsorption geometry. The prospects for using these selection rules to directly determine both the resonance symmetry and the molecular adsorption site are discussed. © 1990 The American Physical Society.