2000A&A...362..774F

A new proposal is put forward for the formation of H₂ from H atoms in the interstellar medium based on the conjecture that efficient production of H^– may take place at the surface of dust grains via attachment of weakly bound surface electrons. Subsequent gas phase associative detachment of H and H^– forms H₂. The model is shown to be compatible with known properties of the interstellar medium in low and high density photodissociation regions and in the interior of dark clouds. Observation of H₂ IR emission in diffuse and dark clouds with the VLT and perhaps also of H^– absorption, using the Far Ultraviolet Spectroscopic Explorer (FUSE), would provide useful tests of the model. The chief source of uncertainty in this proposal lies in the lack of data for binding energies of electrons to grains and thus in the feasibility of the H^– surface formation process.