H-2 formation in space: a negative ion route?

A new proposal is put forward for the formation of H-2 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 boun d surface electrons. Subsequent gas phase associative detachment of H and H - forms H-2. The model is shown to be compatible with known properties of the interstell ar medium in low and high density photodissociation regions and in the inte rior of dark clouds. Observation of H-2 LR emission in diffuse and dark clo uds with the VLT and perhaps also of H- absorption. using the Far Ultraviol et Spectroscopic Explorer (FUSE), would provide useful tests of the model. The chief source of uncertainty in this proposal lies in the lack of data f or binding energies of electrons to grains and thus in the feasibility of t he H- surface formation process.