Letter to the Editor

Mobility of D atoms on porous amorphous water ice surfaces under interstellar conditions

Université de Cergy-Pontoise & Observatoire de Paris, LERMA, UMR 8112 du CNRS, 95000 Cergy-Pontoise, France e-mail: Elie.Matar@u-cergy.fr

Received: 20 June 2008
Accepted: 23 October 2008

Abstract

Aims. The mobility of H atoms on the surface of interstellar dust grains at low temperature is still a matter of debate. In dense clouds, the hydrogenation of adsorbed species (i.e., CO), as well as the subsequent deuteration of the accreted molecules depend on the mobility of H atoms on water ice. Astrochemical models widely assume that H atoms are mobile on the surface of dust grains even if controversy still exists. We present here direct experimental evidence of the mobility of H atoms on porous water ice surfaces at 10 K.

Methods. In a UHV chamber, O₂ is deposited on a porous amorphous water ice substrate. Then D atoms are deposited onto the surface held at 10 K. Temperature-Programmed Desorption (TPD) is used and desorptions of O₂ and D₂ are simultaneously monitored.

Results. We find that the amount of O₂ that desorbs during the TPD diminishes if we increase the deposition time of D atoms. O₂ is thus destroyed by D atoms even though these molecules have previously diffused inside the pores of thick water ice. Our results can be easily interpreted if D is mobile at 10 K on the water ice surface. A simple rate equation model fits our experimental data and best fit curves were obtained for a D atom diffusion barrier of 22 ± 2 meV. Therefore hydrogenation can take place efficiently on interstellar dust grains. These experimental results are consistent with most calculations and validate the hypothesis used in several models.