Molecular Hydrogen Formation in the Interstellar Medium

We have developed a model for molecular hydrogen formation underastrophysically relevant conditions. This model takes fully into account thepresence of both physisorbed and chemisorbed sites on the surface, allowsquantum mechanical diffusion as well as thermal hopping for absorbed H-atoms,and has been benchmarked versus recent laboratory experiments on H2 formationon silicate surfaces. The results show that H2 formation on grain surface isefficient in the interstellar medium up to some 300K. At low temperatures(<100K), H2 formation is governed by the reaction of a physisorbed H with achemisorbed H. At higher temperatures, H2 formation proceeds through reactionbetween two chemisorbed H atoms. We present simple analytical expressions forH2 formation which can be adopted to a wide variety of surfaces once theirsurfaces characteristics have been determined experimentally.Comment: 4 pages, 1 figur