The Origin of Positive Ions and Excited Neutrals in Electron Stimulated Desorption from Alkali Halides

Abstract

The interaction of electron and photon beams with insulating materials is of substantial current interest. Alkali halides are the most ionic of the ionic insulators, and they have comparatively simple geometric and electronic structures, thus they are useful prototypes for basic studies. When energetic electrons or photons are incident on alkali halides, particles are desorbed with high efficiency. The flux of particles leaving the bombarded region includes ground-state neutrals, excited-state neutrals, and positive ions. The vast majority of the desorbed particles are ground-state neutral atoms and molecules, which are ejected via bulk-defect mediated processes [l]. The ground-state yields can be remarkably large - for 1 keV electrons incident on NaCl at T=300 C, roughly 5 Na atoms and 5 CI atoms are desorbed per incident electron [2,3]. The desorption mechanisms for the ground-state particles have been qualitatively understood for some time. The initial excitation caused by the incident radiation is electron-hole generation in the bulk. The holes (neutral halogen atoms) are incorporated into stable defects called H centers which can diffuse to the surface, where neutral halogen atom desorption can occur. Neutral alkali atoms can be formed on the surface by re-combination of electrons and alkali ions, and these neutral atoms can then thermally desorb. This process appears to be limited by the diffusion of electrons (as F centers) to the surface [4]. The very large yields of ground state neutrals are related to the fact that the initial excitation results in a stable defect in the bulk, and desorption can occur much later after defect diffusion to the surface. The time delay due to the diffusion of defects to the surface has been directly observed in experiments using pulsed electron beams [4,5].