Electronic sputtering of solid O 2

Abstract

The bombardment of low temperature condensed gas solids by MeV light ions produces electronic excitations that can decay non-radiatively, transferring kinetic energy to the lattice and causing ejection (sputtering) of atoms. The experimental data for the sputtering yield Y of solid O 2 and N 2 over a range of d E/d x are proportional to (d E/d x) 2, where d E/d x is the energy deposited per unit path length. Parametrizing an analytical thermal spike model with constant track radius gave satisfactory agreement with the data [Johnson et al., Phys. Rev. B 44, (1991) 14]. However, molecular dynamics calculations for solid O 2 indicate that Y is proportional to d E/d x at high d E/d x for constant track radius and that the energy transport processes differ from those assumed in spike models. Here we propose that the quadratic dependence in the experimental data is due to a track radius that increases with d E/d x, opposite to the dependence predicted by the Bohr adiabatic radius. This radius is determined by fast energy transport processes prior to the principal energy release due to lattice motion: e.g. by hole repulsion and diffusion, by cooling electrons, or by excitation transport. Energy transport for a vibrationally excited track was examined, and it was found that sputtering was inefficient for vibrational excitation of solid O 2.