Low-energy electronic excitation in atomic collision cascades: A nonlinear transport model

A computer simulation model is presented which allows one to incorporate low-energy electronic excitation into the molecular dynamics computer simulation of atomic collision cascades in metals. The model treats the electronic energy losses experienced by all moving atoms as a source term for electronic excitation energy, which is assumed to spread around the original point of excitation with a diffusivity D. In order to acknowledge (i) the large temperature gradients and (ii) the local lattice disorder within the cascade volume, the electronic heat diffusivity is allowed to vary as a function of space and time, thus leading to a strongly nonlinear diffusion of electronic excitation energy. The corresponding diffusion equation is developed and numerically solved for an exemplary collision cascade initiated by the impact of a 5 keV silver atom onto an Ag(111) surface. It is shown that electron surface temperatures of several thousand kelvin can be reached at times after the impact at which most emission of surface particles occurs. This excitation may therefore influence the ionization or excitation of such sputtered species.