  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 H2 formation
on silicate surfaces. The results show that H2 formation on grain surface is
efficient in the interstellar medium up to some 300K. At low temperatures
(<100K), H2 formation is governed by the reaction of a physisorbed H with a
chemisorbed H. At higher temperatures, H2 formation proceeds through reaction
between two chemisorbed H atoms. We present simple analytical expressions for
H2 formation which can be adopted to a wide variety of surfaces once their
surfaces characteristics have been determined experimentally.
