H 2 formation on interstellar grains in different physical regimes

An analysis of the kinetics of H 2 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.