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.
