Linear-to-quadratic transition in electronically stimulated sputtering of solidN2andO2

Electronic excitations produced by MeV protons and helium ions lead to ejection of molecules from solid ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$. The yield is shown to be determined by the near-surface excitation density, rather than the primary ionization cross section. It is linear in the excitation density at low excitation densities and quadratic at high excitation densities. The linear yield can be described by discrete, nonradiative transfers of electronic energy into kinetic energy (‘‘spikes’’) that utilize ≲12% of the total electronic energy deposited for ${\mathrm{N}}_2$ and ≲37% for ${\mathrm{O}}_2$. A statistical model is used to calculate the transition from low to high excitation density due to the overlap of either Maxwellian or non-Maxwellian spikes. It is found that the separate ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ data sets are consistent in this model, that excitations do not diffuse very far from where they are created, that the linear-to-quadratic transition is described better by the non-Maxwellian spikes, and that the energy derived in the linear regime is adequate to account for the nonlinear yields.