PNNL

Abstract

The low-energy, electron-stimulated production of molecular oxygen from pure amorphous solid water (ASW) films and ASW films co-dosed with H2O2 is investigated. Layered films of H216O and H218O are used to determine the spatial profile of the reactions in the films leading to O2. The O2 yield is dose-dependent, indicating that precursors are involved in the O2 production. For temperatures below ~80 K, the O2 yield at steady state is relatively low and nearly independent of temperature. At higher temperatures, the yield increases rapidly. The O2 yield is enhanced from H2O2-dosed water films, but the experiments show that H2O2 is not the final precursor in the reactions leading to O2. Instead, a stable precursor for O2 is produced through a multi-step reaction sequence probably involving the reactions of OH radicals to produce H2O2 and then HO2. The O2 is produced in a non-thermal reaction from the HO2. For relatively thick films, the reactions leading to O2 occur at or near the ASW/vacuum interface. However, the electronic excitations which initiate the reactions occur over a larger range in the film. A kinetic model which qualitatively accounts for all of the observations is presented.