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Aims. The mobility of H atoms on the surface of interstellar dust grains atlow temperature is still a matter of debate. In dense clouds, the hydrogenationof adsorbed species (i.e., CO), as well as the subsequent deuteration of theaccreted molecules depend on the mobility of H atoms on water ice.Astrochemical models widely assume that H atoms are mobile on the surface ofdust grains even if controversy still exists. We present here directexperimental evidence of the mobility of H atoms on porous water ice surfacesat 10 K. Methods. In a UHV chamber, O2 is deposited on a porous amorphous waterice substrate. Then D atoms are deposited onto the surface held at 10 K.Temperature-Programmed Desorption (TPD) is used and desorptions of O2 and D2are simultaneously monitored. Results. We find that the amount of O2 thatdesorb 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 previouslydiffused inside the pores of thick water ice. Our results can be easilyinterpreted if D is mobile at 10 K on the water ice surface. A simple rateequation model fits our experimental data and best fit curves were obtained fora D atoms diffusion barrier of 22(+-)2 meV. Therefore hydrogenation can takeplace efficiently on interstellar dust grains. These experimental results arein line with most calculations and validate the hypothesis used in severalmodels.

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