VUV photo-processing of PAH cations: quantitative study on the
  ionization versus fragmentation processes
  Interstellar polycyclic aromatic hydrocarbons (PAHs) are strongly affected by
the absorption of vacuum ultraviolet (VUV) photons in the interstellar medium
(ISM), yet the branching ratio between ionization and fragmentation is poorly
studied. This is crucial for the stability and charge state of PAHs in the ISM
in different environments, affecting in turn the chemistry, the energy balance,
and the contribution of PAHs to the extinction and emission curves. We studied
the interaction of PAH cations with VUV photons in the 7-20 eV range from the
synchrotron SOLEIL beamline, DESIRS. We recorded by action spectroscopy the
relative intensities of photo-fragmentation and photo-ionization for a set of
eight PAH cations ranging in size from 14 to 24 carbon atoms, with different
structures. At photon energies below ~13.6 eV fragmentation dominates for the
smaller species, while for larger species ionization is immediately competitive
after the second ionization potential (IP). At higher photon energies, all
species behave similarly, the ionization yield gradually increases, leveling
off between 0.8 and 0.9 at ~18 eV. Among isomers, PAH structure appears to
mainly affect the fragmentation cross section, but not the ionization cross
section. We also measured the second IP for all species and the third IP for
two of them, all are in good agreement with theoretical ones confirming that
PAH cations can be further ionized in the diffuse ISM. Determining actual PAH
dication abundances in the ISM will require detailed modeling. Our measured
photo-ionization yields for several PAH cations provide a necessary ingredient
for such models.
