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