Electronically stimulated sputtering and luminescence from solid argon

Electronic excitation of condensed-gas solids by MeV light ions results both in luminescence and in kinetic ejection of atoms and molecules from the surface of the solid. This work reveals that ejection of Ar atoms from electronically excited solid argon films arises from two nonradiative repulsive dissociation steps in the deexcitation decay sequence which contains the major radiative transition. A hole-diffusion model with nonradiative quenching at the metal substate on which the films are grown accounts for the dependence of the sputtering and luminescence on film thickness and gives an estimate of 230 Å for the hole-diffusion length. The hole-diffusion model also explains the nearly linear dependence of the sputtering and luminescence yields on electronic stopping cross section for high-velocity incident ions. Intentionally added ${\mathrm{O}}_2$ and ${\mathrm{N}}_2$ impurities reduce the hole-diffusion length. These reductions provide values for effective reaction volumes, $k_{+}$ ${\tau }_{+}$, of 7×${10}^{\mathrm{-}21}$ ${\mathrm{cm}}^3$ for ${\mathrm{O}}_2$ and 3×${10}^{\mathrm{-}21}$ ${\mathrm{cm}}^3$ for ${\mathrm{N}}_2$, where $k_{+}$ is the interaction rate constant of the holes with the impurity and ${\tau }_{+}$ is the lifetime of holes in pure films. The ${\mathrm{O}}_2$ and ${\mathrm{N}}_2$ impurities also quench nonmobile luminescent excimers, ${\mathrm{Ar}}_2^{\mathrm{*}}$, over large distances: 21 Å for ${\mathrm{O}}_2$ and 11 Å for ${\mathrm{N}}_2$. The magnitude of the sputtering yield characterizes the vacuum interface as primarily hole reflecting, but with a surface-hole-trapping rate about 6 times that of the bulk. Surface ${\mathrm{O}}_2$ impurity greatly enhances surface trapping of free holes. Sputtering of ${\mathrm{Ar}}_2^{\mathrm{*}}$ dimers is evident from the observation of 11.3-eV luminescence in the region in front of the argon films. The dimers are emitted with ∼0.8 eV of kinetic energy. This is consistent with the repulsive decay forming an energetic ${\mathrm{Ar}}^{\mathrm{*}}$, which forms ${\mathrm{Ar}}_2^{\mathrm{*}}$ upon exiting or, possibly, the ejection of an ${\mathrm{Ar}}_2^{\mathrm{*}}$ during trapping at the surface.