The Electronic Desorption of Excited Alkali Atoms from Alkali Halide Surfaces

Abstract

Alkali halides have long been known to erode under electron and photon bombardment. The detailed nature of the electronic mechanisms responsible for the erosion remains, however, a matter of vigorous discussion. In order to elucidate some of these mechanisms we present recent measurements of the incident beam energy dependence of excited alkali neutral desorption arising from electron and photon bombardment of alkali halide surfaces. In all cases, the yields for excited alkali neutrals are measured to be much greater than those obtained for positive ion ejection due to electron or photon stimulated desorption. Substrate core hole formation is seen to play an important role in the initial energy transfer process. In each case only the lowest resonance excited states are observed. The energy dependent yield of excited neutrals arising from electron bombardment is found to differ markedly from the corresponding photon bombardment data. Present theories dealing with (a) the production and migration of defects, and (b) the ejection of ions, are not adequate to explain the desorption of neutral alkalis in excited states. In both the incident electron and photon cases, our observations suggest microscopic models which take into account the mechanisms of initial electronic excitation of the surface, subsequent ejection of a surface particle and possible electron exchange as the particle leaves the surface.