Surface formation of CO2 ice at low temperatures

The surface formation of CO2 at low temperatures through the reaction CO + OH and direct dissociation of the resulting HO–CO complex is shown by hydrogenation of a CO:O2 ice mixture. Such a binary ice is not fully representative for an interstellar ice, but the hydrogenation of O2 ice produces OH radicals, which allows the investigation of the interstellar relevant CO + OH solid state reaction under fully controlled laboratory conditions. Similar recent astrophysical ice studies have focused on the investigation of isolated surface reaction schemes, starting from the hydrogenation of pure ices, like solid CO or O2. For such ices, no CO2 formation is observed upon H-atom exposure. The hydrogenation of binary ice mixtures presented here allows to investigate for the first time the influence of the presence of other species in the ice on the pure ice reaction shemes. Mixtures of CO:O2 are deposited on a substrate in an ultra high vacuum setup at low temperatures (15 and 20 K) and subsequently hydrogenated. The ice is monitored by means of Reflection Absorption InfraRed Spectroscopy (RAIRS). Results show that solid CO2 is formed in all studied CO:O2 mixtures under our laboratory conditions. Within the experimental uncertainties no dependency on ice temperature or composition is observed. The laboratory results show a correlation between the formation of CO2 and H2O, which is consistent with the astronomical observation of solid CO2 in water-rich environments. The results also show that the contemporary presence of CO and O2 molecules in the ice influences the final product yields of the separate CO + H (H2CO, CH3OH) and O2+ H (H2O2 and H2O) channels, even though the formation rates are not significantly affected.