H₂ formation on interstellar grains in different physical regimes

An analysis of the kinetics of H₂ formation on interstellar dust grains is presented using rate equations. It is shown that semi-empirical expressions that have appeared in the literature represent two different physical regimes. In particular, it is shown that the expression given by Hollenbach, Werner & Salpeter applies when high flux, or high mobility, of H atoms on the surface of a grain makes it very unlikely that H atoms evaporate before they meet each other and recombine. The expression of Pirronello et al. - deduced on the basis of accurate measurements of a realistic dust analogue - applies to the opposite regime (low coverage and low mobility). The implications of this analysis for the understanding of the processes dominating in the interstellar medium are discussed.