Deuterium enrichment of ammonia produced by surface N+H/D addition reactions at low temperature

TL;DR

This study investigates how ammonia and its deuterated forms form on cold surfaces, revealing a preference for deuterium enrichment due to higher D atom sticking probabilities, relevant for understanding interstellar chemistry.

Contribution

It provides new insights into low-temperature surface reactions, showing deuterium enrichment mechanisms in ammonia formation relevant to astrochemical models.

Findings

Ammonia and isotopologues form only up to 15 K.

Deuterium enrichment favors D atoms due to higher sticking probabilities.

Results help interpret ammonia deuterium fractionation in space.

Abstract

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.