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The deuteration of ammonia by D atoms has been investigated experimentally inthe sub-monolayer regime on realistic analogues of interstellar dust grainsurfaces. About 0.8 monolayer of solid NH3 was deposited on top of an oxidizedgraphite surface held at 10 K, partly covered with ASW ice. Ammonia ice issubsequently exposed to D atoms for different exposure times using adifferentially pumped beam-line. The deuteration experiments of ammonia werecarried out by mass spectroscopy and temperature programmed desorption (TPD)technique. The experimental results showed the formation of three isotopologueammonia species by direct exothermic H-D substitution surface reactions:NH3+D->NH2D+H, NH2D+D->NHD2+H, and NHD2+D->ND3+H. The formation of thedeuterated isotopologues NH2D, NHD2, and ND3 at low surface temperature (10 K)is likely to occur through quantum tunneling process on the oxidized graphitesurface. A kinetic model taking into account the diffusion of D atoms on thesurface is developed to estimate the width and the hight of the activationenergy barriers for the successive deuteration reactions of ammonia species byD atoms. Identical control experiments were performed using CH3OH and D atoms.The deuteration process of solid methanol is ruled by H abstraction and Daddition mechanism, and is almost five orders of magnitude faster than ammoniadeuteration process.

Deuteration of ammonia with D atoms on oxidized partly ASW covered graphite surface