Molecular dynamics study of non-equilibrium energy transport from a cylindrical track: Part II: Spike models for sputtering yield

Abstract

Thermal spike models have been used to calculate the yields for electronic sputtering of condensed-gas solids by fast ions. In this paper molecular dynamics (MD) calculations are carried out to describe the evolution of solid Ar and O2 following the excitation of a cylindrical track in order to test spike models. The calculated sputtering yields were found to depend linearly on the energy deposition per unit path length, dE/dx, at the highest dE/dx. This is in contrast to the spike models and the measured yields for a number of condensed-gas solids which depend quadratically on dE/dx at high dE/dx. In paper I [E.M. Bringa, R.E. Johnson, Nucl. Instr. and Meth. B 143 (1998) 513] we showed that the evolution of energy from the cylindrical track was, typically, not diffusive, as assumed in the spike models. Here we show that it is the description of the radial transport and the absence of energy transport to the surface, rather than the treatment of the ejection process, that accounts for the difference between the analytic spike models and the MD calculations. Therefore, the quadratic dependence on dE/dx of the measured sputtering yield reflects the nature of the energizing process rather than the energy transport. In this paper we describe the details of the sputtering process and compare the results here for crystalline samples to the earlier results for amorphous solids.