Lonely adatoms in space

TL;DR

This paper explores the failure of standard rate equations in modeling surface reactions on interstellar dust grains, providing a new derivation of hydrogen recombination rates that accounts for grain size and confinement effects.

Contribution

It introduces a scaling-based derivation of hydrogen recombination rates on dust grains, explaining rate equation failures and identifying a logarithmic correction in certain regimes.

Findings

Rate equations fail for small grains due to confinement effects.

A new scaling derivation clarifies hydrogen recombination rates.

Logarithmic correction applies when desorption limits the reaction.

Abstract

There is a close relation between the problems of second layer nucleation in epitaxial crystal growth and chemical surface reactions, such as hydrogen recombination, on interstellar dust grains. In both cases standard rate equation analysis has been found to fail because the process takes place in a confined geometry. Using scaling arguments developed in the context of second layer nucleation, I present a simple derivation of the hydrogen recombination rate for small and large grains. I clarify the reasons for the failure of rate equations for small grains, and point out a logarithmic correction to the reaction rate when the reaction is limited by the desorption of hydrogen atoms (the second order reaction regime).