Effect of surface H₂on molecular hydrogen formation on interstellar grains

We investigate how the existence of hydrogen molecules on grain surfaces may affect H2 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 H2 on H2 formation efficiency strongly depends on the diffusion barriers of H2 on grain surfaces. Diffuse cloud models that do not consider surface H2 predict that H atom recombination efficiency is above 0.5 over a grain temperature (T) range 10 and 14 K. The adopted H2 diffusion barriers in diffuse cloud models that consider surface H2 are 80% H2 desorption energies so that H2 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 ≤T≤ 14 K. Translucent cloud model results show that H2 formation efficiency is not affected by the existence of surface H2 if the adopted average H2 diffusion barrier on grain surfaces is low (194 K) so that H2 can diffuse rapidly on grain surfaces. However, the recombination efficiency can drop to below 0.002 atT≥ 10 K if higher average H2 diffusion barrier is used (255 K) in translucent cloud models.