Surface chemistry in the interstellar medium - I - H2 formation by
  Langmuir-Hinshelwood and Eley-Rideal mechanisms
  H2 formation remains a major issue for the understanding of interstellar
physics. We investigate H2 formation in the interstellar medium at the light of
the most recent experimental and theoretical data. We implemented detailed H2
formation mechanisms on grains surface in the Meudon PDR code : i)
Langmuir-Hinshelwood mechanism taking into account the contribution of the
different sizes of dust grains in the diffusion processes and ii) the
Eley-Rideal mechanism. We show that, thanks to these processes, H2 can be
formed even in regions where dust temperature is larger than 25 K. We also show
that formation by Eley-Rideal mechanism can be a significant source of heating
of the gas. We derive line intensities for various astrophysical conditions.
Such an approach results in an enhanced H2 formation rate compared to the
standard formation determined by observations in absorption in the UV. We
derive some H2 line intensities for isobaric and isochoric models.
