Formation of localized states and electronic excitation in solids during low-energy particle bombardment

Abstract

The excitation of deep holes (2s, 2p, in Al) in collision cascades is known to be responsible for emission of high-energy secondary electrons from solids bombarded by fast particles. The excitation occurs above certain impact energy threshold ( ≈ 800 eV in the case of Al) which is given by the minimum interatomic distance in the binary atomic collision needed for inner shell-valence band energy crossing ( ≈ 0.5 Å in Al). Experiments indicate however, that electrons may be emitted from solids far below the deep-hole excitation threshold. The classical valence electron-particle binary collision mechanism which is responsible for most of the particle electronic energy losses yields only low energy electrons which cannot overcome the work function of solids. We have found out that in collision cascades the deformation of valence electron clouds in more complex collision can lead to localized levels below the bottom of the conduction band of a metal. When these levels loose electrons during dynamical particle-particle interactions their subsequent filling by Auger processes could lead to electron emission. Using a cluster of 123 Al atoms with 3s and 3p overlapping Gaussian orbitals and using the pseudopotential of solid Al we have calculated the localization and energies of these new time-dependent electronic states for some collision configurations. We have also estimated the probability of the hole occupation of such states induced by the dynamics of particle cascades.