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Abstract: UV irradiation of simple ices is proposed to efficiently produce complexorganic species during star- and planet-formation. Through a series oflaboratory experiments, we investigate the effects of the H2O concentration,the dominant ice constituent in space, on the photochemistry of more volatilespecies, especially CH4, in ice mixtures. In the experiments, thin ~40 ML icemixtures, kept at 20-60 K, are irradiated under ultra-high vacuum conditionswith a broad-band UV hydrogen discharge lamp. Photodestruction cross sectionsof volatile species CH4 and NH3 and production efficiencies of new speciesC2H6, C2H4, CO, H2CO, CH3OH, CH3CHO and CH3CH2OH in water-containing icemixtures are determined using reflection-absorption infrared spectroscopyduring irradiation and during a subsequent slow warm-up. The four major effectsof increasing the H2O concentration are 1 an increase of the destructionefficiency of the volatile mixture constituent by up to an order of magnitudedue to a reduction of back reactions following photodissociation, 2 a shift toproducts rich in oxygen e.g. CH3OH and H2CO, 3 trapping of up to a factor offive more of the formed radicals in the ice and 4 a disproportional increasein the diffusion barrier for the OH radical compared to the CH3 and HCOradicals. The radical diffusion temperature dependencies are consistent withcalculated H2O-radical bond strengths. All the listed effects are potentiallyimportant for the production of complex organics in H2O-rich icy grain mantlesaround protostars and should thus be taken into account when modeling icechemistry.

Author: Karin I. Oberg CfA, Ewine F. van Dishoeck Leiden University, Harold Linnartz Leiden University, Stefan Andersson SINTEF


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