The Interaction of H Atoms with Interstellar Dust Particles: Models

Summary

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.