The catalytic role of water in the photochemistry of ammonia ice: From diluted to concentrated phase

Abstract

Using infrared spectroscopy as an in situ probe for reactions occurring in the solid phase, we investigated the influence of water molecules on the photochemistry of ammonia ices. Experiments were carried out in diluted and concentrated phases and between 3 and 130 K. We showed that the photolysis of NH3-H2O (2 per cent of H2O) ices using continuous radiation from 115 to 400 nm produces NH2OH as the main photoproduct, but also that such a photoinduced reaction strongly depends on both the initial ice temperature and the environment where the primary reactants NH3 and H2O are trapped. Our experimental results highlight the catalytic role played by H2O molecules in enhancing the formation yield of NH2 during the photolysis process through the NH3 + OH → NH2 + H2O hydrogen abstraction reaction, which is only favored at low temperatures in the range of 3-60 K. During heating of such irradiated ammonia-water ices, the amount of NH2OH keeps rising while that of NH2, is greatly reduced only from 70 K onwards. These behaviours are attributed to the competition that occurs between NH2 formation from the NH3 + OH reaction and its consumption from the NH2 + OH radical recombination. These results might explain the variable abundances of NH2 and NH3 provided by previous astronomical observations, where the NH2/NH3 ratio ranges from 0.02 to 0.5 depending on the regions of the interstellar medium that were analysed.