A Proposed Modification of the Rate Equations for Reactions on Grain Surfaces

The rate equations currently utilized for grain reactions in gas-grain chemical models of interstellar clouds are inappropriate under certain conditions because of the discrete nature of grain particles. Appropriate modifications are suggested, and calculated surface abundances for various molecules are tested against Monte Carlo calculations for three simple systems. In the first system, the gas is comprised of only O and H atoms that accrete on grain surfaces and subsequently react to form only the diatomic molecules H2, OH, and O2. In the second system, N atoms are also present in the gas, and the grain chemistry leads to more molecules, including the polyatomic species NH3 and H2O. In the third and final system, the gas contains H2, and the contribution of this species to grain chemistry is considered. The modifications to the rate equations lead to reasonable agreement with the results of Monte Carlo simulations for all three systems. These modifications can then be used in time-dependent gas-grain chemical models until a more detailed but usable theory becomes available.