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

Authors:
Open Access
Publisher website
Google Scholar
Abstract
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 modiÐcations are suggested, and calculated surface abundances for various molecules are tested against Monte Carlo calculations for three simple systems. In the Ðrst 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 OH, and In the second system, N atoms are also present in the gas, and the grain H 2 , O 2. chemistry leads to more molecules, including the polyatomic species and In the third and NH 3 H 2 O. Ðnal system, the gas contains and the contribution of this species to grain chemistry is considered. H 2 , The modiÐcations to the rate equations lead to reasonable agreement with the results of Monte Carlo simulations for all three systems. These modiÐcations can then be used in time-dependent gas-grain chemical models until a more detailed but usable theory becomes available.