Laboratory Photo-chemistry of PAHs: Ionization versus Fragmentation
  Interstellar polycyclic aromatic hydrocarbons (PAHs) are expected to be
strongly processed by vacuum ultraviolet photons. Here, we report experimental
studies on the ionization and fragmentation of coronene (C24H12), ovalene
(C32H14) and hexa-peri-hexabenzocoronene (HBC; C42H18) cations by exposure to
synchrotron radiation in the range of 8--40 eV. The results show that for small
PAH cations such as coronene, fragmentation (H-loss) is more important than
ionization. However, as the size increases, ionization becomes more and more
important and for the HBC cation, ionization dominates. These results are
discussed and it is concluded that, for large PAHs, fragmentation only becomes
important when the photon energy has reached the highest ionization potential
accessible. This implies that PAHs are even more photo-stable than previously
thought. The implications of this experimental study for the photo-chemical
evolution of PAHs in the interstellar medium are briefly discussed.
