Modified rate equations revisited. A corrected treatment for diffusive reactions on grain surfaces

Abstract

It is well known that the rate equations for diffusive reactions on grain surfaces can be inappropriate under certain circumstances because they do not take the discrete nature of grains into account. A previous modification of the rate equations developed by us to treat surface chemistry on grains more accurately contains an error in the probability of evaporation from grain surfaces. With the rate of evaporation handled correctly, we show for a simple system in which only O and H atoms accrete on grain surfaces and react to form H2, OH, and O2 that the modified rate method is in reasonable agreement with a corrected Monte Carlo approach through 20 K and in excellent agreement with a new master equation approach at 10 K.