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

Henda Chaabouni (1, 2) , Marco Minissale (3) , Saoud Baouche (1, 2) , Francois Dulieu (1, 2)

1 LERMA (UMR_8112) - Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique
2 LERMA - LERMA Cergy
3 PIIM - Physique des interactions ioniques et moléculaires

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 NH 3 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: NH 3 + D → NH 2 D + H, NH 2 D + D → NHD 2 + H, and NHD 2 + D → ND 3 + H. The formation of the deuterated isotopologues NH 2 D, NHD 2 , and ND 3 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 CH 3 OH 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.

Domaines: Physique [physics]; Planète et Univers [physics]

Henda Chaabouni, Marco Minissale, Saoud Baouche, Francois Dulieu. Deuteration of ammonia with D atoms on oxidized partly ASW covered graphite surface. 2017. ⟨hal-02982177⟩