Ground-State and Excited Atom Production by Electron and Ion Bombardment of NaCl and CaF2

Abstract

For alkali and alkali earth haliaes, ion, electron and photon bombardment are efficient processes leading to continuous surface erosion [1,2]. While sputtering of metals under ion bombardment is well understood in terms of a collision cascade emission mechanism based on momentum transfer due to elastic collisions [3], for electron or photon bombardment no such processes can account for any particle emission due to the negligible momentum transfer. Electronic processes have been proposed to explain such electron or photon induced desorption processes [4,5]. Selective emission of the halogen atoms and molecules is ascribed to the H-centre migration model leading to the formation of a metal overlayer on the surface. If the vapour pressure of the alkali/alkali earth metal layer at the target temperature is high enough these excess metal atoms will desorb thermally, thus leading to continuous particle emission. Otherwise the excess metal layer will eventually prevent any further halogen emission and the emission process will be only of transient nature [6]. We have compared the particle emission processes for ion and electron bombardment of NaCl and CaF2 to differentiate between a collision cascade induced contribution and one due to Desorption Induced by Electronic Transitions (DIET) under ion bombardment and, in addition, to compare DIET processes under ion and electron bombardment to obtain a better understanding of the desorption mechanisms involved.