Molecule Formation In and On Grains.I: Physical Regimes

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We study molecular hydrogen formation in and on solids. We construct a model with surface sites and bulk sites capable of describing (1) the motion and exchange of H and H₂ between surface and bulk, (2) the recombination of H and dissociation of H₂ in and on the solid, and (3) the injection of H from the gas phase and the loss of H and H₂ from the solid. The basic physical processes include thermally activated reactions, collisionally induced reactions, and tunneling reactions. Our main application is to the astronomical problem of H₂ formation on grains in space but the model has more general applicability. We investigate the steady-state H and H₂ concentrations in and on the solid when gas phase atoms or ions stick to the surface or penetrate the body of the grain. The model identifies ranges of physical parameters for which the solid becomes saturated (surface and/or bulk) with particles (H and/or H₂) and facilitates the calculation of the efficiency of molecule formation (the fraction of the gas phase atoms that leave as molecules). These solutions are highly degenerate in the sense that they depend only on a small number of dimensionless parameters. We find that a variety of recombination pathways operate under a broad range of conditions. As an example we study H₂ formation in and on carbon grains.