Ion energy distributions from photon- and electron-stimulated desorption. II. The quasiclassical final state and reneutralization.

Abstract

Franck-Condon overlap integrals are calculated to fit ion energy distributions (IED's) resulting from photon- or electron-stimulated desorption. The WKB or quasiclassical trajectory approximation is used to compute the final-state wave function. The final-state potential consists of a repulsive exponential and an image term. The calculated IED's compare well with those found previously using the reflection or classical trajectory approximation. Both methods agree well with a wide range of experimental IED's all of which are mildly asymmetric Gaussian functions of the ion energy. It is shown how these two approximations are related through the classical limit of the final-state wave function. In addition, the quasiclassical wave function is generalized to include an optical potential giving rise in a natural way to the reneutralization reduction factor. The Gaussian asymmetry in the IED's is usually to the high-energy side reflecting a ``normal'' exponential repulsion. However, in a few anomalous cases the Gaussian is skewed to the low-energy side of the IED peak. This anomaly cannot be explained by expected influences on the initial-state wave function such as anharmonicities. On the other hand, the reneutralization factor, when it predominates, provides a natural explanation for this effect. In addition, this reneutralization factor can cause significant shifts in the IED peak, away from the Franck-Condon peak.