The formation of interstellar H2 on amorphous silicate grains

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.