On the role of electronic friction and electron promotion in kinetic excitation of solids

The kinetic electronic excitation of a solid due to impact of energetic particles onto the surface is investigated using a combination of molecular dynamics and analytical theory. Two different mechanisms are considered as a source for kinetic excitation, namely electronic friction (i.e. the inelastic energy loss experienced by all moving particles) and electron promotion in close collisions. Both mechanisms deliver a time and position dependent source term which is introduced into a nonlinear diffusive treatment of the transport of electronic excitation energy within the solid. Results show that the majority of the kinetic energy originally imparted to the surface is dissipated into electronic excitation rather than collision dynamics. Analysis of the temporal evolution reveals electron promotion to be the dominant source of excitation at short times (<100 fs) after the projectile impact, while electronic friction clearly dominates the excitation on the picosecond time scale.