Monte Carlo simulation of electron emission induced by swift highly charged ions: beyond the linear response approximation

Numerical simulations of ion induced electron emission from solids mostly use the first order Born approximation within the dielectric formalism to describe valence electron excitation. As a result, the yield of emitted electrons is found to scale with the square of the projectile charge QP in contrast to experimental findings obtained with carbon targets. Since similar deviations from QP2 scaling were observed for the electronic stopping power, at least a part of this deviation must be related to primary ion-electron interaction, for which an alternative description needs to be developed. We thus present here a distorted wave approach for the modelling of primary interaction, which can be expected to give better results in view of its success in describing ion-atom collisions at large impact velocity. Keeping the same description of the electron transport through the target, we show that both the electron yield and the stopping power ratios (with respect to the same quantities for C6+), as a function of the projectile charge, are better reproduced by this alternative approach. We show that low energy electron excitation is responsible for the deviation from the QP2 scaling. We also analyse the effect of the transport on the primary electrons. This distorted wave approach successfully explains the shape of the ratio of energy differential spectra for two different QP obtained in earlier experiment for Al and C. Furthermore, we predict a different behaviour of the forward and backward electron emission with respect to QP in qualitative agreement with experimental results. (authors)