H-2 FORMATION ON INTERSTELLAR GRAINS IN DIFFERENT PHYSICAL REGIMES

An analysis of the kinetics of H-2 formation on interstellar dust grai ns is presented using rate equations. It is shown that semi-empirical expressions that have appeared in the literature represent two differe nt physical regimes. In particular, it is shown that the expression gi ven by Hollenbach, Werner & Salpeter applies when high flux, or high m obility, of H atoms on the surface of a grain makes it very unlikely t hat H atoms evaporate before they meet each other and recombine. The e xpression of Pirronello et al. - deduced on the basis of accurate meas urements of a realistic dust analogue - applies to the opposite regime (low coverage and low mobility). The implications of this analysis fo r the understanding of the processes dominating in the interstellar me dium are discussed.