Effect of surface H$_2$ on molecular hydrogen formation on interstellar grains

We investigate how the existence of hydrogen molecules on grain surfaces may affect H$_2$ formation efficiency in diffuse and translucent clouds. Hydrogen molecules are able to reduce the desorption energy of H atoms on grain surfaces in models. The detailed microscopic Monte Carlo method is used to perform model simulations. We found that the impact of the existence of H$_2$ on H$_2$ formation efficiency strongly depends on the diffusion barriers of H$_2$ on grain surfaces. Diffuse cloud models that do not consider surface H$_2$ predict that H atom recombination efficiency is above 0.5 over a grain temperature (T) range 10 K and 14 K. The adopted H$_2$ diffusion barriers in diffuse cloud models that consider surface H$_2$ are 80$\%$ H$_2$ desorption energies so that H$_2$ can be trapped in stronger binding sites. Depending on model parameters, these diffuse cloud models predict that the recombination efficiency is between nearly 0 and 0.5 at 10 K $\leq$ T $\leq$ 14 K. Translucent cloud model results show that H$_2$ formation efficiency is not affected by the existence of surface H$_2$ if the adopted average H$_2$ diffusion barrier on grain surfaces is low (194 K) so that H$_2$ can diffuse rapidly on grain surfaces. However, the recombination efficiency can drop to below 0.002 at T $\geq$ 10 K if higher average H$_2$ diffusion barrier is used (255 K) in translucent cloud models.

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