The low-energy, electron-stimulated production of molecular oxygen from thin amorphous solid water (ASW) films adsorbed on Pt(111) is investigated. For ASW coverages less than similar to 60 ML, the O-2 electron-stimulated desorption (ESD) yield depends on coverage in a manner that is very similar to the H-2 ESD yield. In particular, both the O-2 and H-2 ESD yields have a pronounced maximum at similar to 20 ML due to reactions at the Pt/water interface. The O-2 yield is dose dependent and several precursors (OH, H2O2, and HO2) are involved in the O-2 production. Layered films of (H2O)-O-16 and (H2O)-O-18 are used to profile the spatial distribution of the electron-stimulated reactions leading to oxygen within the water films. Independent of the ASW film thickness, the final reactions leading to O-2 occur at or near the ASW/vacuum interface. However, for ASW coverages less than similar to 40 ML, the results indicate that dissociation of water molecules at the ASW/Pt interface contributes to the O-2 production at the ASW/vacuum interface presumably via the generation of OH radicals near the Pt substrate. The OH (or possibly OH-) segregates to the vacuum interface where it contributes to the reactions at that interface. The electron-stimulated migration of precursors to the vacuum interface occurs via transport through the hydrogen bond network of the ASW without motion of the oxygen atoms. A simple kinetic model of the nonthermal reactions leading to O-2, which was previously used to account for reactions in thick ASW films, is modified to account for the electron-stimulated migration of precursors. (c) 2006 American Institute of Physics.

The low-energy, electron-stimulated production of molecular oxygen from pure amorphous solid water (ASW) films and ASW films codosed with H(2)O(2) is investigated. Layered films of H(2)(16)O and H(2)(18)O are used to investigate the reaction mechanisms for O(2) production and the spatial profile of the reactions within the films. The O(2) yield is dose-dependent, indicating that precursors are involved in the O(2) production. For temperatures below approximately 80 K, the O(2) yield at steady state is relatively low and nearly independent of temperature. At higher temperatures, the yield increases rapidly. The O(2) yield is enhanced from H(2)O(2)-dosed water films, but the experiments show that H(2)O(2) is not the final precursor in the reactions leading to O(2). Instead, a stable precursor for O(2) is produced through a multistep reaction sequence probably involving the reactions of OH radicals to produce H(2)O(2) and then HO(2). The O(2) is produced in a nonthermal reaction from the HO(2). For relatively thick films, the reactions leading to O(2) 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.