Surface Reactions and Desorption by Electron Attachment

Abstract

Electrons can attach to atoms and molecules in different ways depending on the charge transfer and charge stabilization mechanism involved [1]. The additional electron may be captured as it collides with the target or it may be supplied by an anion or metastable atom or molecule interacting with the target. The initial anion formed in this electron transfer process is usually in a transient short-lived (10−10–10−16 sec) state which underdoes subsequent modifications. The formation of transient anions can be divided into two broad categories [2]. If the additional electron occupies a previously unfilled orbital of a target atom or molecule in its ground state, the transitory state is referred to as a single-particle resonance. The term “shape” resonance applies more specifically when temporary trapping of the electron is due to the shape of the electron-target potential. When the transitory anion is formed by two electrons occupying previously unfilled orbitals, the resonance is called “core-excited” or may be referred to as a two-particle, one-hole state. In the case of the latter, the electron is captured by the positive electron affinity of an electronically excited state of the target with or without the assistance of an angular momentum barrier. In a single-particle resonance the centrifugal barrier set by the colliding electron is usually responsible for the capture, but the additional electron may also be temporarily trapped by exciting nuclear motion in a molecule. In all cases, a potential well is formed which temporarily captures the additional electron. The dimensions of this well are such that it accommodates electrons having wavefunctions corresponding to low energies.