Abstract
We present a computer simulation model for the space- and time-resolved calculation of electronic excitation energy densities in atomic collision cascades. The model treats electronic friction as well as electron promotion as a source term of electronic energy that is carried away from the original point of excitation according to a nonlinear diffusion equation. While the frictional source is treated within the Lindhard model of electronic stopping, electron promotion is described using diabatic correlation curves derived from ab initio molecular orbital energy level calculations in combination with the Landau–Zener curve crossing model. Results calculated for two selected collision cascades show that the electron promotion mechanism may contribute significantly to the excitation energy density in the cascade volume, giving rise to distinct peaks of the local electron temperature at the surface. This contribution is essentially restricted to the first 100 fs after the projectile impact and may therefore be of significance for either external or internal kinetic electron emission. At later times, where the bombardment-induced particle kinetics lead to the sputter ejection of material from the surface, the excitation is shown to be primarily governed by electronic friction. This finding is important in light of excitation and ionization probabilities of sputtered particles.
Highlights
The bombardment of a metal surface with keV ions initiates a complex series of atomic collisions in a near-surface region
The temporal and spatial evolution of the electronic excitation energy density will be discussed in respect to the ionization probability of sputtered particles and kinetic electron emission
To the atomic particles, which are diagrammed as small colored balls, the electronic subsystem is represented by a gas cloud whose colorization reflects the momentary local excitation energy density as calculated by Eq(9)
Summary
The bombardment of a metal surface with keV ions initiates a complex series of atomic collisions in a near-surface region. Due to the fact that an ab-initio calculation of large scale particle dynamics, which would directly include electronic excitations, is still too complex and not feasible for a sputtering scenario, several attempts [5, 6, 26, 27, 34] have been made to incorporate electronic excitation processes into standard molecular dynamics computer simulations In all of these approaches electrons only play a passive role either as a static medium acting as a friction force, which is calculated within the framework of the LSS-model or similar approaches employing local electron densities [24], leading to a slowing down of moving atomic particles, or as a non-relevant byproduct accompanying the deep level core hole generation in a hard binary collision event. First results obtained for one selected ion impact event (5-keV Ag → Ag(111)) have revealed interesting temporal dynamics of the substrate excitation at the surface: Immediately following the projectile impact excitation energy densities are reached that correspond to electron temperatures of several thousand Kelvin This initial excitation rapidly dissipates due to the onset of fast diffusion in the ideal metal lattice. Once the model is completed, the calculations will be expanded to a set of many different impact points in order to assess the statistics of the excitation processes
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