The chemistry of vibrationally excited H2 in the interstellar medium
  The internal energy available in vibrationally excited H2 molecules can be
used to overcome or diminish the activation barrier of various chemical
reactions of interest for molecular astrophysics. In this article we
investigate in detail the impact on the chemical composition of interstellar
clouds of the reactions of vibrationally excited H2 with C+, He+, O, OH, and
CN, based on the available chemical kinetics data. It is found that the
reaction of H2 (v>0) and C+ has a profound impact on the abundances of some
molecules, especially CH+, which is a direct product and is readily formed in
astronomical regions with fractional abundances of vibrationally excited H2,
relative to ground state H2, in excess of 10^(-6), independently of whether the
gas is hot or not. The effects of these reactions on the chemical composition
of the diffuse clouds zeta Oph and HD 34078, the dense PDR Orion Bar, the
planetary nebula NGC 7027, and the circumstellar disk around the B9 star HD
176386 are investigated through PDR models. We find that formation of CH+ is
especially favored in dense and highly FUV illuminated regions such as the
Orion Bar and the planetary nebula NGC 7027, where column densities in excess
of 10^(13) cm^(-2) are predicted. In diffuse clouds, however, this mechanism is
found to be not efficient enough to form CH+ with a column density close to the
values derived from astronomical observations.
