Electronically induced sputtering of ground and excited state alkali atoms from alkali halide crystals

Abstract

The following processes lead to electronic desorption from alkali halides during low-energy electron or photon bombardment: a) direct bond breaking, and b) collision induced formation of defects in the crystals. The former process may depend on defect creation but involves primarily the desorption of adsorbates bonded on the surface of the crystal and also may be resonant for the case of photon bombardment. The latter process generally leads to the desorption of the constituents of the alkali halide crystals (alkali and halogen atoms). In this paper the authors concentrate on processes leading to the desorption of alkali atoms in either the ground state, or excited state, therefore b)-type processes. Experimental methods used to investigate the desorbing species were quadrupole mass spectroscopy, laser induced fluorescence spectroscopy, and fluorescence spectroscopy. Transmission optical absorption spectroscopy performed simultaneously with desorption measurements of neutral ground state atoms was used to characterize the mobility and stability of defect clusters created in the crystals and to correlate them with the desorption yield of the alkali atoms. Fluorescence spectroscopy measurements show that a significant part of the alkali atoms desorb in an excited state. We discuss a new model which explains the yield of excited alkali atoms at moderate temperatures by means of a surface chemistry process arising from defect migration to the surface.