  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.
