Water formation at low temperatures by surface O₂ hydrogenation II: the reaction network

Water is abundantly present in the Universe. It is the main component of interstellar ice mantles and a key ingredient for life. Water in space is mainly formed through surface reactions. Three formation routes have been proposed in the past: hydrogenation of surface O, O₂, and O₃. In a previous paper [Ioppolo et al., Astrophys. J., 2008, 686, 1474] we discussed an unexpected non-standard zeroth-order H₂O₂ production behaviour in O₂ hydrogenation experiments, which suggests that the proposed reaction network is not complete, and that the reaction channels are probably more interconnected than previously thought. In this paper we aim to derive the full reaction scheme for O₂ surface hydrogenation and to constrain the rates of the individual reactions. This is achieved through simultaneous H-atom and O₂ deposition under ultra-high vacuum conditions for astronomically relevant temperatures. Different H/O₂ ratios are used to trace different stages in the hydrogenation network. The chemical changes in the forming ice are followed by means of reflection absorption infrared spectroscopy (RAIRS). New reaction paths are revealed as compared to previous experiments. Several reaction steps prove to be much more efficient (H + O₂) or less efficient (H + OH and H₂ + OH) than originally thought. These are the main conclusions of this work and the extended network concluded here will have profound implications for models that describe the formation of water in space.