  The surface formation of NH3 and its deuterated isotopologues - NH2D, NHD2,
and ND3 - is investigated at low temperatures through the simultaneous addition
of hydrogen and deuterium atoms to nitrogen atoms in CO-rich interstellar ice
analogues. The formation of all four ammonia isotopologues is only observed up
to 15 K, and drops below the detection limit for higher temperatures.
Differences between hydrogenation and deuteration yields result in a clear
deviation from a statistical distribution in favour of deuterium enriched
species. The data analysis suggests that this is due to a higher sticking
probability of D atoms to the cold surface, a property that may generally apply
to molecules that are formed in low temperature surface reactions. The results
found here are used to interpret ammonia deuterium fractionation as observed in
pre-protostellar cores.
