H2 formation via the UV photo-processing of a-C:H nano-particles
  Context. The photolysis of hydrogenated amorphous carbon, a-C(:H), dust by UV
photon-irradiation in the laboratory leads to the release of H2 as well as
other molecules and radicals. This same process is also likely to be important
in the interstellar medium. Aims. To investigate molecule formation arising
from the photo-dissociatively-driven, regenerative processing of a-C(:H) dust.
Methods. We explore the mechanism of a-C(:H) grain photolysis leading to the
formation of H2 and other molecules/radicals. Results. The rate constant for
the photon-driven formation of H2 from a-C(:H) grains is estimated to be
2x10^-17 cm^3 s^-1. In intense radiation fields photon-driven grain
decomposition will lead to fragmentation into daughter species rather than H2
formation. Conclusions. The cyclic re-structuring of arophatic a-C(:H)
nano-particles appears to be a viable route to formation of H2 for low to
moderate radiation field intensities (1 < G_0 < 10^2), even when the dust is
warm (T ~ 50 - 100 K).
