Formation, trapping, and ejection of radiolytic $O2$ from ion-irradiated water ice studied by sputter depth profiling

We report experimental studies of $100keV$ $Ar+$ ion irradiation of ice leading to the formation of molecular oxygen and its trapping and ejection from the surface, at temperatures between 80 and $150K$⁠. The use of a mass spectrometer and a quartz-crystal microbalance and sputter depth profiling at $20K$ with low energy Ar ions allowed us to obtain a consistent picture of the complex radiolytic mechanism. We show that the dependence of $O2$ sputtering on ion fluence is mainly due to the buildup of trapped $O2$ near the surface. A small proportion of the $O2$ is ejected above $130K$ immediately upon creation from a precursor such as OH or $H2O2$⁠. The distribution of trapped oxygen peaks at or near the surface and is shallower than the ion range. Measurements of sputtering of $H2$ help to elucidate the role of this molecule in the process of $O2$ formation: out-diffusion leading to oxygen enrichment near the surface. The competing phenomena of OH diffusion away from the ion track and hydrogen escape from the ice and their temperature dependence are used to explain the finding of opposite temperature dependencies of $O2$ and $H2O2$ synthesis. Based on the new data and understanding, we discuss the application of our findings to ices in the outer solar system and interstellar space.