Collisional and electronic processes under ion, electron and photon bombardment of alkali and alkaline-earth halides

Abstract

The interaction of energetic ions or electrons with alkali halides and alkaline-earth halides results in elastic and inelastic energy transfer to the near surface region. The emission of secondary sputtered and desorbed particles has been studied to identify these processes. In particular, we have compared the emission of neutral ground state and excited alkali and alkaline-earth atoms from NaCl, LiF and CaF 2. The intensity of the sputtered and desorbed particles and their velocity distributions have been measured as a function of the target temperature between room temperature and 400 ° C. For NaCl under ion bombardment neutral ground state Na atoms desorb predominantly (ion-beam-induced) thermally, even at room temperature, while Ca atoms from CaF 2 desorb mainly thermally above 300 °C and purely collisional below 250 °C. For 400 eV electron bombardment desorption of thermal neutral ground state atoms is observed whenever a thermal contribution is present under ion bombardment. Excited Na and Ca atoms with energies in the eV range have been observed for ion bombardment. For electron bombardment excited Na has been identified with thermal velocities at all target temperatures. Contrary, excited Ca was not found even at target temperatures where substantial ground state Ca is desorbed. For NaCl as well as CaF 2 and electron as well as ion bombardment inelastic processes play an important role for particle ejection above a certain target temperature.