  Hydroxylamine (NH2OH) is one of the potential precursors of complex
pre-biotic species in space. Here we present a detailed experimental study of
hydroxylamine formation through nitric oxide (NO) surface hydrogenation for
astronomically relevant conditions. The aim of this work is to investigate
hydroxylamine formation efficiencies in polar (water-rich) and non-polar
(carbon monoxide-rich) interstellar ice analogues. A complex reaction network
involving both final (N2O, NH2OH) and intermediate (HNO, NH2O, etc.) products
is discussed. The main conclusion is that hydroxylamine formation takes place
via a fast and barrierless mechanism and it is found to be even more abundantly
formed in a water-rich environment at lower temperatures. In parallel, we
experimentally verify the non-formation of hydroxylamine upon UV photolysis of
NO ice at cryogenic temperatures as well as the non-detection of NC- and
NCO-bond bearing species after UV processing of NO in carbon monoxide-rich
ices. Our results are implemented into an astrochemical reaction model, which
shows that NH2OH is abundant in the solid phase under dark molecular cloud
conditions. Once NH2OH desorbs from the ice grains, it becomes available to
form more complex species (e.g., glycine and beta-alanine) in gas phase
reaction schemes.
