Astrochemical relevance of VUV ionization of large PAH cations
  As a part of interstellar dust, polycyclic aromatic hydrocarbons (PAHs) are
processed by the interaction with vacuum ultraviolet (VUV) photons that are
emitted by hot young stars. This interaction leads to the emission of the
well-known aromatic infrared bands but also of electrons, which can
significantly contribute to the heating of the interstellar gas.Our aim is to
investigate the impact of molecular size on the photoionization properties of
cationic PAHs.Methods. Trapped PAH cations of sizes between 30 and 48 carbon
atoms were submitted to VUV photons in the range of 9 to 20 eV from the DESIRS
beamline at the synchrotron SOLEIL. All resulting photoproducts including
dications and fragment cations were mass-analyzed and recorded as a function of
photon energy.Photoionization is found to be predominant over dissociation at
all energies, which differs from an earlier study on smaller PAHs. The
photoionization branching ratio reaches 0.98 at 20 eV for the largest studied
PAH. The photoionization threshold is observed to be between 9.1 and 10.2 eV,
in agreement with the evolution of the ionization potential with size.
Ionization cross sections were indirectly obtained and photoionization yields
extracted from their ratio with theoretical photoabsorption cross sections,
which were calculated using time-dependent density functional theory. An
analytical function was derived to calculate this yield for a given molecular
size.Large PAH cations could be efficiently ionized in H i regions and provide
a contribution to the heating of the gas by photoelectric effect. Also, at the
border of or in H ii regions, PAHs could be exposed to photons of energy higher
than 13.6 eV. Our work provides recipes to be used in astronomical models to
quantify these points.
