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Water is the main component of interstellar ice mantles, is abundant in thesolar system and is a crucial ingredient for life. The formation of thismolecule in the interstellar medium cannot be explained by gas-phase chemistryonly and its surface hydrogenation formation routes at low temperatures (O, O2,O3 channels) are still unclear and most likely incomplete. In a previous paperwe discussed an unexpected zeroth-order H2O production behavior in O2 icehydrogenation experiments compared to the first-order H2CO and CH3OH productionbehavior found in former studies on hydrogenation of CO ice. In this paper weexperimentally investigate in detail how the structure of O2 ice leads to thisrare behavior in reaction order and production yield. In our experiments Hatoms are added to a thick O2 ice under fully controlled conditions, while thechanges are followed by means of reflection absorption infrared spectroscopy(RAIRS). The H-atom penetration mechanism is systematically studied by varyingthe temperature, thickness and structure of the O2 ice. We conclude that thecompetition between reaction and diffusion of the H atoms into the O2 iceexplains the unexpected H2O and H2O2 formation behavior. In addition, we showthat the proposed O2 hydrogenation scheme is incomplete, suggesting thatadditional surface reactions should be considered. Indeed, the detection ofnewly formed O3 in the ice upon H-atom exposure proves that the O2 channel isnot an isolated route. Furthermore, the addition of H2 molecules is found notto have a measurable effect on the O2 reaction channel.

Water formation at low temperatures by surface O2 hydrogenation I: characterization of ice penetration