Electron-Stimulated Production of Molecular Oxygen in Amorphous Solid Water

• Published in: The journal of physical chemistry. B Vol. 110; no. 6; pp. 2723 - 2731
• Main Authors: Petrik, Nikolay G, Kavetsky, Alexander G, Kimmel, Greg A
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 16-02-2006
• Subjects: Electrons; Oxygen - chemistry; Temperature; Time Factors; Water - chemistry
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp055173v

The low-energy, electron-stimulated production of molecular oxygen from pure amorphous solid water (ASW) films and ASW films codosed with H2O2 is investigated. Layered films of H2 16O and H2 18O are used to investigate the reaction mechanisms for O2 production and the spatial profile of the reactions within the films. 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 multistep reaction sequence probably involving the reactions of OH radicals to produce H2O2 and then HO2. The O2 is produced in a nonthermal 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 that initiate the reactions occur over a larger range in the film. A kinetic model that qualitatively accounts for all of the observations is presented.