Energy thresholds and delayed emission for electron-stimulated desorption of neutral ground- and excited-state Li atoms from lithium fluoride

Abstract

We have bombarded single-crystal surfaces of LiF with 5–500 eV electrons at different target temperatures (300–800 K) and the temperature dependence of the yield of ground- and excited-state Li atoms was studied. Measuring the electron energy threshold for desorption of Li∗ we found an onset for desorption of Li∗ around 60 eV, which correlates well with the Li+ core-exciton levels of Li+ in LiF. Based on these findings we propose a Knotek-Feibelman-like mechanism for the desorption of excited Li atoms from LiF initiated by Li core electron excitations (core excitons). On the other hand, we found the threshold for desorption of Li° atoms to be much lower, around 12 eV. This is explained with a process involving valence band excitation and subsequent F- and H-center formation. In related experiments we have studied the delayed emission of ground-state atoms after turning off the electron beam, which can be explained in terms of F-center production by the primary electron beam, Li metal colloid formation and the subsequent evaporation and diffusion of Li to the surface, resulting in metal desorption.