  The deuteration of ammonia by D atoms has been investigated experimentally in
the sub-monolayer regime on realistic analogues of interstellar dust grain
surfaces. About 0.8 monolayer of solid NH3 was deposited on top of an oxidized
graphite surface held at 10 K, partly covered with ASW ice. Ammonia ice is
subsequently exposed to D atoms for different exposure times using a
differentially pumped beam-line. The deuteration experiments of ammonia were
carried out by mass spectroscopy and temperature programmed desorption (TPD)
technique. The experimental results showed the formation of three isotopologue
ammonia 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 the
deuterated isotopologues NH2D, NHD2, and ND3 at low surface temperature (10 K)
is likely to occur through quantum tunneling process on the oxidized graphite
surface. A kinetic model taking into account the diffusion of D atoms on the
surface is developed to estimate the width and the hight of the activation
energy barriers for the successive deuteration reactions of ammonia species by
D atoms. Identical control experiments were performed using CH3OH and D atoms.
The deuteration process of solid methanol is ruled by H abstraction and D
addition mechanism, and is almost five orders of magnitude faster than ammonia
deuteration process.
