Molecular dynamics studies of minicascades in electronically stimulated sputtering of condensed-gas solids

Abstract

An analytic model and molecular dynamics (MD) simulations are used to describe the collisional energy transport and sputtering initiated by a 1–4 eV atom or molecule in low temperature, solid Ar, O2, and N2. In these systems energetic exothermic processes, such as repulsive relaxation events following electronic excitation by a fast ion, an electron or an ultra violet photon, can give kinetic energy to an atom or molecule initiating a sequence of low energy collisions, a minicascade. When such an event occurs near the surface in a low-temperature, condensed-gas solid, atomic or molecular ejection can result. Using MD calculations the moving particles are tracked and the energy and angular spectra of the ejected particles determined. For a distribution of excitations which is uniform with depth, the average number of particles ejected is shown to be proportional to the average initial kinetic energy divided by the sublimation energy. The proportionality constant is only weakly dependent on the condensed-gas solid, since, for the molecular solids studied, the amount of the initial energy transferred into vibrational excitation of the ejecta is small. Therefore, measured electronic-sputtering yields provide an estimate of the energy release in energetic, nonradiative relaxation events. The ejecta exhibit a nearly E−2 kinetic energy spectra for recoil energies, E, much greater than the sublimation energy. The MD calculation of the yields and of the ejecta energy and angular distributions are interpreted here using an analytic model of a cascade of collisions in which the collisions in the solid are not assumed to be binary.