| Abstract
| - A model for H2 formation by reaction of H-atoms with the surface of carbonaceous grains is developed. Such reactions, which involve interstitial H-atoms at low grain temperature and H-atoms bonded to -CH, -CH2 and -CH3 groups at higher temperature, occur promptly on the collision of an H-atom with a grain surface. The rate of hydrogen formation and the efficiency of reaction depend on both gas kinetic and dust temperature. The rate coefficient R (cm3 s−1) for H2 formation using this prompt-reaction model (PRM) can be calculated from laboratory data and is consistent with measured values of R. The dependence of R on ambient conditions in interstellar clouds can be easily estimated from this prompt-reaction model. R is found to be strongly dependent on gas kinetic temperature Tk and can exceed 10−17 cm3 s−1, even for warm dust, when Tk≿300 K.
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