THE INFLUENCE OF DEFECTS AND DEFECT CLUSTERS ON ALKALI ATOM DESORPTION STIMULATED BY LOW-ENERGY-ELECTRON BOMBARDMENT OF ALKALI-HALIDES

An important challenge in studies of electronically induced desorption on alkali halides is to determine the degree to which the desorption rate of alkali atoms as a function of dose and temperature is influenced by alkali island formation on the surface, alkali colloid formation in the bulk, and individual defect diffusion from the bulk to the surface. To address this problem, we report measurements of transmission optical absorption which gives direct information on defect, cluster and surface island concentrations, and of electron bombardment induced alkali desorption yields performed simultaneously on NaCl, NaF, and LiF at temperatures between 27 and 400-degrees-C. These experiments provide insight into the formation of surface and bulk agglomerates which in turn act as sources of desorbing alkali atoms. Our data support a physical picture where bombardment induced F-centers lead to the formation of F-center aggregates in the bulk and alkali metal clusters on the surface at temperatures around room temperature. At higher temperatures alkali metal desorption during electron bombardment is dominated by the emission of weakly bound single alkali atoms, and alkali atoms from alkali metal clusters on the surface of the crystals. After the cessation of the electron beam, the desorption yield is controlled by the thermal stability of metallic colloids which have been formed in the bulk during electron bombardment due to the temperature dependent higher mobility of the F-centers.