Kinetic Monte Carlo simulations of the grain-surface back-diffusion effect

TL;DR

This study uses kinetic Monte Carlo simulations to quantify how back-diffusion on interstellar dust grains reduces chemical reaction rates, providing data to improve rate-equation models.

Contribution

It offers the first detailed quantification of back-diffusion effects on dust grain reaction rates and provides fitted functions for model integration.

Findings

Back-diffusion can reduce reaction rates by up to a factor of 5.

Grain morphology and coverage significantly influence back-diffusion effects.

Fitted logarithmic functions enable incorporation of back-diffusion into rate-equation models.

Abstract

Back-diffusion is the phenomenon by which random walkers revisit binding sites on a lattice. This phenomenon must occur on interstellar dust particles, slowing down dust-grain reactions, but it is not accounted for by standard rate-equation models. Microscopic kinetic Monte Carlo models have been used to investigate the effect of back-diffusion on reaction rates on interstellar dust grains. Grain morphology, size, and grain-surface coverage were varied and the effects of these variations on the magnitude of the back-diffusion effect were studied for the simple H+H reaction system. This back-diffusion effect is seen to reduce reaction rates by a maximum factor of ~5 for the canonical grain of 10$^6$ binding sites.The resulting data were fit to logarithmic functions that can be used to reproduce the effects of back-diffusion in rate-equation models.