Master Equation for Hydrogen Recombination on Grain Surfaces

Recent experimental results on the formation of molecular hydrogen onastrophysically relevant surfaces under conditions similar to those encounteredin the interstellar medium provided useful quantitative information about theseprocesses. Rate equation analysis of experiments on olivine and amorphouscarbon surfaces provided the activation energy barriers for the diffusion anddesorption processes relevant to hydrogen recombination on these surfaces.However, the suitability of rate equations for the simulation of hydrogenrecombination on interstellar grains, where there might be very few atoms on agrain at any given time, has been questioned. To resolve this problem, weintroduce a master equation that takes into account both the discrete nature ofthe H atoms and the fluctuations in the number of atoms on a grain. Thehydrogen recombination rate on microscopic grains, as a function of grain sizeand temperature, is then calculated using the master equation. The results arecompared to those obtained from the rate equations and the conditions underwhich the master equation is required are identified.Comment: Latex document. 14 pages of text. Four associated figs in in PS format on separate files that are "called-in" the LaTeX documen