Molecular hydrogen formation in the interstellar medium

We have developed a model for molecular hydrogen formation under astrophysically relevant conditions. This model takes fully into account the presence of both physisorbed and chemisorbed sites on the surface, allows quantum mechanical diffusion as well as thermal hopping for absorbed H atoms, and has been benchmarked versus recent laboratory experiments on H-2 formation on silicate surfaces. The results show that H-2 formation on grain surfaces is efficient in the interstellar medium up to some 300 K. At low temperatures (less than or equal to100 K), H-2 formation is governed by the reaction of a physisorbed H with a chemisorbed H. At higher temperatures, H-2 formation proceeds through a reaction between two chemisorbed H atoms. We present simple analytical expressions for H-2 formation that can be adopted to a wide variety of surfaces once their surface characteristics have been determined experimentally.