ELECTRONICALLY INDUCED SPUTTERING OF GROUND AND EXCITED-STATE ALKALI ATOMS FROM ALKALI-HALIDE CRYSTALS

The following processes lead to electronic desorption from alkali hali des during low-energy electron or photon bombardment: a) direct bond b reaking, and b) collision induced formation of defects in the crystals . The former process may depend on defect creation but involves primar ily the desorption of adsorbates bonded on the surface of the crystal and also may be resonant for the case of photon bombardment. The latte r 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 a toms in either the ground state, or excited state, therefore b)-type p rocesses. Experimental methods used to investigate the desorbing speci es were quadrupole mass spectroscopy, laser induced fluorescence spect roscopy, and fluorescence spectroscopy. Transmission optical absorptio n spectroscopy performed simultaneously with desorption measurements o f 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 spec troscopy 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 .