On the encounter desorption of hydrogen atoms on an ice mantle

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 H_(2 )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.