Computer simulation of low-energy electronic excitations in atomic collision cascades

We present a computer simulation of low-energy electronic excitations that are created in atomic collision cascades initiated by the impact of energetic particles onto a solid surface. In order to render a chemically inert system, the self-bombardment of a silver (1 1 1) surface with Ag atoms is simulated. In the model, the atomic motion following the particle impact is described by a classical molecular dynamics approach. The transfer of kinetic into electronic excitation energy is described in terms of a friction-like electronic energy loss experienced by every moving atom in the solid, thus leading to a space and time dependent density of electron-hole pair excitation energy generated in the course of the collision cascade. This energy is assumed to spread around the point of original excitation with a diffusion coefficient D and to equipartition in the Ag sp band according to a Fermi distribution characterized by an electronic temperature T-e(r, t). It is shown that for reasonable values of D the electronic energy deposited at the surface can be substantial, thus leading to transient electronic surface temperatures reaching several thousands of Kelvin which, for instance, can influence the ionization probabilities of sputtered atoms. (C) 2004 Elsevier B.V. All rights reserved.