Chemistry on interstellar grains

Abstract

In this review, our current knowledge of the chemistry that occurs on cold interstellar grain surfaces is examined. The formation of molecular hydrogen from hydrogen atoms is emphasized. Various methods of modeling diffusive reactions on grain surfaces are illustrated, including the rate equation method, and two basic stochastic approaches. The results of a detailed Monte Carlo method - the continuous-time random-walk approximation - in which the problem of random walk in two dimensions is treated exactly, are shown and analyzed for the case of H2 formation. Both homogeneous surfaces such as olivine and amorphous carbon and inhomogeneous surfaces are studied. Results from this method show that (a) inhomogeneous surfaces allow efficient H2 formation over wider temperature ranges than do homogeneous surfaces, and (b) the reaction efficiency declines for grains smaller than a certain size, but the effect is smaller for inhomogeneous surfaces.