Photo-processing of astro-PAHs
  Polycyclic aromatic hydrocarbons (PAHs) are key species in astrophysical
environments in which vacuum ultraviolet (VUV) photons are present, such as
star-forming regions. The interaction with these VUV photons governs the
physical and chemical evolution of PAHs. Models show that only large species
can survive. However, the actual molecular properties of large PAHs are poorly
characterized and the ones included in models are only an extrapolation of the
properties of small and medium-sized species. We discuss here experiments
performed on trapped ions including some at the SOLEIL VUV beam line DESIRS. We
focus on the case of the large dicoronylene cation, C48H20+ , and compare its
behavior under VUV processing with that of smaller species. We suggest that
C2H2 is not a relevant channel in the fragmentation of large PAHs. Ionization
is found to largely dominate fragmentation. In addition, we report evidence for
a hydrogen dissociation channel through excited electronic states. Although
this channel is minor, it is already effective below 13.6 eV and can
significantly influence the stability of astro-PAHs. We emphasize that the
competition between ionization and dissociation in large PAHs should be further
evaluated for their use in astrophysical models.
