Auger Spectroscopy and Electronically-stimulated Surface Processes

Abstract

Electronic excitationsin adsorbate layers stimulate desoprtion and dissociation of adsorbed melecules as well as chemical reactions between adsorbates. The highest-probability stimulated processes produce neutral desorbates and determine how surface composition is altered by elctron or photon radiation. A basic understanding has emerged, due largely to laser resonance-enhanced multiphoton ionization (REMPI) experiments, which provide quantum-state resolution of the gas-phase products.Auger phenomena enter this understanding in several ways. For example, CVV Auger spectroscopy determines the screened hole-hole interaction, U, in adsorbates, which in turn provides insight into the degree of charge-transfer screening from the substrate. In those systems where screening charge is used in excitation Auger decay, screening direcly determines the lifetime, which in turn can exponentially affect the yield. Reductions in screening, e.g. induced by coadsorption of electro-negative species, thus can result in giant yield enhancements. As separate issues, a finite U may prevent the fast resonant decay and this increase the yield from two-hole excitations, as has been suggested for NO2 dissociation on Pt(111), or may assist in the localization (self-trapping) of two-hole excitations in dense adsorbate layers, as apparently is the case for NO desorption from the same surface. The latter causes the yields from one- and two-hole excitations to differ in their coverage dependence. Finally, CVV Auger spectroscopy, of course, measures the energies of two-hole excitations, which can be correlated with observed stimulated thresholds.