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At the low temperatures of interstellar dust grains, it is well establishedthat surface chemistry proceeds via diffusive mechanisms of H atoms weaklybound (physisorbed) to the surface. Until recently, however, it was unknownwhether atoms heavier than hydrogen could diffuse rapidly enough oninterstellar grains to react with other accreted species. In addition, modelsstill require simple reduction as well as oxidation reactions to occur ongrains to explain the abundances of various molecules. In this paper weinvestigate O-atom diffusion and reactivity on a variety of astrophysicallyrelevant surfaces (water ice of three different morphologies, silicate, andgraphite) in the 6.5 - 25 K temperature range. Experimental values were used toderive a diffusion law that emphasizes that O atoms diffuse by quantummechanical tunnelling at temperatures as low as 6.5 K. The rate of diffusion oneach surface, based on modelling results, were calculated and an empirical lawis given as a function of the surface temperature. Relative diffusion rates arek_H2Oice > k_sil > k_graph >> k_expected. The implications of an efficientO-atom diffusion over astrophysically relevant time-scales are discussed. Ourfindings show that O atoms can scan any available reaction partners (e.g.,either another H atom, if available, or a surface radical like O or OH) at afaster rate than that of accretion. Also, as dense clouds mature H2 becomes farmore abundant than H and the O/H ratio grows, the reactivity of O atoms ongrains is such that O becomes one of the dominant reactive partners togetherwith H.

Efficient diffusive mechanisms of O atoms at very low temperatures on surfaces of astrophysical interest