2010ApJ...718..832O

UV irradiation of simple ices is proposed to efficiently produce complex organic species during star formation and planet formation. Through a series of laboratory experiments, we investigate the effects of the H₂ O concentration, the dominant ice constituent in space, on the photochemistry of more volatile species, especially CH₄, in ice mixtures. In the experiments, thin (∼40 ML) ice mixtures, kept at 20-60 K, are irradiated under ultra-high vacuum conditions with a broadband UV hydrogen discharge lamp. Photodestruction cross sections of volatile species (CH₄ and NH₃) and production efficiencies of new species (C₂H₆, C₂H₄, CO, H₂CO, CH₃OH, CH₃CHO, and CH₃CH₂OH) in water-containing ice mixtures are determined using reflection-absorption infrared spectroscopy during irradiation and during a subsequent slow warm-up. The four major effects of increasing the H₂ O concentration are: (1) an increase of the destruction efficiency of the volatile mixture constituent by up to an order of magnitude due to a reduction of back reactions following photodissociation, (2) a shift to products rich in oxygen, e.g., CH₃OH and H₂CO, (3) trapping of up to a factor of 5 more of the formed radicals in the ice, and (4) a disproportional increase in the diffusion barrier for the OH radical compared with the CH₃ and HCO radicals. The radical diffusion temperature dependencies are consistent with calculated H₂O-radical bond strengths. All the listed effects are potentially important for the production of complex organics in H₂O-rich icy grain mantles around protostars and should thus be taken into account when modeling ice chemistry.