Two topics of relevance for H2 formation in the interstellar medium are considered: (i) the interaction of H and H-H with a model-graphite surface (Coronene: C24H12), and (ii) H− formation by charge transfer in the interaction of H with a model-silicate surface (MgO{100} representing forsterite: Mg2SiO4{100}). The first topic is related to the frequently invoked Langmuir-Hinshelwood and Eley-Rideal mechanisms for H2 formation near carbonaceous zones of interstellar dust grains. Ab initio calculations based on Density Functional Theory are used. The second topic proposes a new scenario in which the efficient production of H− ions would subsequently enable the formation of H2 via associative detachment. It stems from recent work of the authors on charge transfer between neutral atoms and ionic insulators.

Introduction

The mechanism of H2 formation in the interstellar medium (ISM) is still an open problem. Owing to the temperature and density conditions existing in the ISM, 3-body recombination and radiative association processes in the gas phase are unable to account for actual H2 abundances. The existence of dust particles in the ISM has attracted the attention of astrophysicists as plausible catalysts or mediators of H-H recombination in space (Hollenbach and Salpeter (1970), Hollenbach and Salpeter (1971)). Current knowledge of interstellar dust particles (IDPs) indicates that they have both a carbonaceous and a silicate composition. This has in particular stimulated the investigation of the role graphitic bonds may have on H2 formation as a result of elementary interactions between H atoms and graphite-like surfaces or platelets.