Electronic Excitations and the Formation of Secondary Ions
The formation and the escape of secondary ions are probably the least well understood aspects of the sputtering process. Numerous theoretical arguments that have been proposed to explain the ionization process in sputtering and possibly also in scattering experiments could be divided into two conceptually different groups. In the first approach the dominant role is played by the time dependent electron coupling between the substrate and the particle whereas the substrate dynamics is completely neglected [1–3]. The second approach stresses the importance of substrate electronic excitations due to the collision cascades induced by ion bombardment and due to the electronic losses of the projectile [4–7]. These two approaches will be referred to, in this paper, as frozen and excited substrate approach, respectively. After a short review of theoretical descriptions of the ionization process, we would like to consider some of the recent experiments in terms of these two competing theoretical concepts.
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