  UV irradiation of simple ices is proposed to efficiently produce complex
organic species during star- and planet-formation. Through a series of
laboratory experiments, we investigate the effects of the H2O 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 broad-band 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 H2O 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
five more of the formed radicals in the ice and 4) a disproportional increase
in the diffusion barrier for the OH radical compared to 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.
