Vol 21, No 2

On the encounter desorption of hydrogen atoms on an ice mantle

Qiang Chang, Xu-Li Zheng, Xia Zhang, Dong-Hui Quan, Yang Lu, Qing-Kuan Meng, Xiao-Hu Li, Long-Fei Chen


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.


Keywords astrochemistry — ISM: abundances — ISM: molecules

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