THE FORMATION OF H-2 BY H-ATOM REACTION WITH GRAIN SURFACES

A model for H-2 formation by reaction of I-I-atoms with the surface of carbonaceous grains is developed. Such reactions, which involve inter stitial I-I-atoms at low grain temperature and H-atoms bonded to -CH, -CH2 and -CH3 groups at higher temperature, occur promptly on the coll ision of an I-I-atom with a grain surface. The rate of hydrogen format ion and the efficiency of reaction depend on both gas kinetic and dust temperature. The rate coefficient R (cm(3) s(-1)) for H-2 formation u sing this prompt-reaction model (PRM) can be calculated from laborator y 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 T-k and can exceed 10(-17) cm(3) s(-1), eve n for warm dust, when T-k greater than or equal to 300 K.