Photoinduced polycyclic aromatic hydrocarbon dehydrogenation: The
  competition between H- and H2-loss
  PAHs constitute a major component of the interstellar medium carbon budget,
locking up to 10--20% of the elemental carbon. Sequential fragmentation induced
by energetic photons leads to the formation of new species, including
fullerenes. However, the exact chemical routes involved in this process remain
largely unexplored. In this work, we focus on the first photofragmentation
steps, which involve the dehydrogenation of these molecules. For this, we
consider a multidisciplinary approach, taking into account the results from
experiments, DFT calculations, and modeling using dedicated Monte-Carlo
simulations. By considering the simplest isomerization pathways --- i.e.,
hydrogen roaming along the edges of the molecule --- we are able to
characterize the most likely photodissociation pathways for the molecules
studied here. These comprise nine PAHs with clearly different structural
properties. The formation of aliphatic-like side groups is found to be critical
in the first fragmentation step and, furthermore, sets the balance of the
competition between H- and H2-loss. We show that the presence of trio
hydrogens, especially in combination with bay regions in small PAHs plays an
important part in the experimentally established variations in the odd-to-even
H-atom loss ratios. In addition, we find that, as PAH size increases, H2
formation becomes dominant, and sequential hydrogen loss only plays a marginal
role. We also find disagreements between experiments and calculations for
large, solo containing PAHs, which need to be accounted for. In order to match
theoretical and experimental results, we have modified the energy barriers and
restricted the H-hopping to tertiary atoms. The formation of H2 in large PAHs
upon irradiation appears to be the dominant fragmentation channel, suggesting
an efficient formation path for molecular hydrogen in PDRs.
