Optical absorption spectroscopy of defects in halides

Abstract

Desorption of alkali atoms from, and defects formed in, alkali halide crystals stimulated by low-energy electron bombardment were investigated simultaneously by optical absorption-, mass spectroscopy-, and depth-profile surface sputtering-techniques. These techniques not only provide important information about the type, amount, and the spatial distribution of the defects formed in the alkali halide crystals, but also indicate which processes govern the emission rate of neutral alkali atoms during and after bombardment. The results show that at temperatures near and lower than room temperature, F-centers, small F-center clusters, and alkali metal clusters are formed during electron bombardment of the crystals. At high temperatures (up to 400°C), colloid formation (metallic phase in the bulk of the crystals) takes place due to the higher mobility of the F-centers. The experimental results clearly indicate that the thermal stability of the colloids control the delayed alkali atom desorption kinetics at elevated temperatures. Depth profile measurements provide strong evidence that the metallic phase is on the surface of the crystals, whereas the F-center type defects are formed in the bulk and near the surface of the crystals. The surface clearly acts as an efficient trap for defects. The spatial distributions of point defects and metal were investigated down to temperatures as low as −90°C where F-center mobilities are negligible. The results show significant low-temperature metallization on the surface which clearly proves that long-range hot-hole diffusion takes place during electron bombardment.