Surface Recombination of Hydrogen Molecules

In this paper we reexamine the problem of the formation of molecular hydrogen on the surfaces of dust grains. We discuss a "recombination efficiency" `(T), the fraction of adsorbed atoms which form molecules before evaporating from a surface, as a function of the surface temperature T. For grains with a perfectly regular surface, ` would be small at grain temperatures exceeding about 13 K. We consider an irregular surface with some lattice-defect or impurity sites having enhanced adsorption energies for atomic hydrogen. Our main result is that even a small number of such sites can greatly extend the range of temperatures over which the efficiency of molecular formation is high. On realistic grains ` 1 for all T Tt , where Tt, is some critical value between 25 and 50 K. Since all dust grains in H I regions are likely to be cooler than Tt,, we conclude that ` = 1 for molecular formation on grains in H I regions