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Context. Water together with O2 are important gas phase ingredients to cooldense gas in order to form stars. On dust grains, H2 O is an importantconstituent of the icy mantle in which a complex chemistry is taking place, asrevealed by hot core observations. The formation of water can occur on dustgrain surfaces, and can impact gas phase composition. Aims. The formation ofmolecules such as OH, H2 O, HO2, H2 O2, as well as their deuterated forms andO2 and O3 is studied in order to assess how the chemistry varies in differentastrophysical environments, and how the gas phase is affected by grain surfacechemistry. Methods. We use Monte Carlo simulations to follow the formation ofmolecules on bare grains as well as the fraction of molecules released into thegas phase. We consider a surface reaction network, based on gas phasereactions, as well as UV photo-dissociation of the chemical species. Results.We show that grain surface chemistry has a strong impact on gas phasechemistry, and that this chemistry is very different for different dust graintemperatures. Low temperatures favor hydrogenation, while higher temperaturesfavor oxygenation. Also, UV photons dissociate the molecules on the surface,that can reform subsequently. The formation-destruction cycle increases theamount of species released into the gas phase. We also determine the timescales to form ices in diffuse and dense clouds, and show that ices are formedonly in shielded environments, as supported by observations.

Water formation on bare grains: When the chemistry on dust impacts interstellar gas