On the Encounter Desorption of Hydrogen Atoms on Ice Mantle

TL;DR

This paper investigates how hydrogen atoms desorb from ice mantles on interstellar grains through encounter desorption, affecting surface chemistry and molecule formation in cold space environments.

Contribution

It introduces the H atom encounter desorption mechanism into models, showing its impact on surface hydrogen abundance and chemical pathways.

Findings

Encounter desorption reduces surface H abundance.

It hinders hydrogenation product formation like methanol.

Facilitates bonding of C, O, N atoms on grain surfaces.

Abstract

At low temperatures (10 K), hydrogen atoms can diffuse quickly on grain ice mantles and frequently encounter hydrogen molecules, which cover a notable fraction of grain surface. The desorption energy of H atoms on H2 substrates is much less than that on water ice. The H atom encounter desorption mechanism is adopted to study the enhanced desorption of H atoms on H2 substrates. Using a small reaction network, we show that the steady-state surface H abundances predicted by the rate equation model that includes H atom encounter desorption agree reasonably well with the results from the more rigorous microscopic Monte Carlo method. For a full gas-grain model, H atom encounter desorption can reduce surface H abundances. Therefore, if a model adopts the encounter desorption of H atoms, it becomes more difficult for hydrogenation products such as methanol to form, but it is easier for C, O and N atoms to bond with each other on grain surfaces.