The formation of interstellar H2 on amorphous silicate grains

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Abstract
A mechanism by which H2 is formed on interstellar amorphous silicate grains is proposed. The mechanism differs from previous proposals in the method of stabilization of the reacting complex, in which the stabilization energy is transferred to a surface band. The reaction to form H2 is promoted by H atom quantum mechanical tunnelling. The model predicts that the H2 on formation is ejected into the gas kinetically and vibrationally excited but rotationally cool. Subsequent collisions thermalize the kinetic energy and populate the rotational levels. The consequences of these predictions for the observed H2 excitation, for interstellar chemistry, and for H2 re-formation behind fast shocks are briefly explored.

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