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

TL;DR

This study provides experimental evidence that D atoms are mobile on porous amorphous water ice surfaces at 10 K, supporting the assumption that H atoms can also diffuse and participate in surface chemistry in interstellar environments.

Contribution

First direct experimental demonstration of D atom mobility on porous water ice at interstellar temperatures, validating key assumptions in astrochemical models.

Findings

D atoms are mobile on water ice at 10 K.

Diffusion barrier for D atoms is approximately 22 meV.

O2 molecules are destroyed by D atoms diffusing inside ice pores.

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 desorb 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 atoms diffusion barrier of 22(+-)2 meV. Therefore hydrogenation can take place efficiently on interstellar dust grains. These experimental results are in line with most calculations and validate the hypothesis used in several models.