Photo-stability of super-hydrogenated PAHs determined by action
  spectroscopy experiments
  We have investigated the photo-stability of pristine and super-hydrogenated
pyrene cations C$_{16}$H$_{10+m}^+, m = 0,6, \mathrm{\ or\ } 16$) by means of
gas-phase action spectroscopy. Optical absorption spectra and photo-induced
dissociation mass spectra are presented. By measuring the yield of
mass-selected photo-fragment ions as a function of laser pulse intensity, the
number of photons (and hence the energy) needed for fragmentation of the carbon
backbone was determined. Backbone fragmentation of pristine pyrene ions
(C$_{16}$H$_{10}^+$) requires absorption of three photons of energy just below
3 eV, whereas super-hydrogenated hexahydropyrene (C$_{16}$H$_{16}^+$) must
absorb two such photons and fully hydrogenated hexadecahydropyrene
(C$_{16}$H$_{26}^+$) only a single photon. These results are consistent with
previously reported dissociation energies for these ions. Our experiments
clearly demonstrate that the increased heat capacity from the additional
hydrogen atoms does not compensate for the weakening of the carbon backbone
when pyrene is hydrogenated. In photodissociation regions, super-hydrogenated
Polycyclic Aromatic Hydrocarbons (PAHs) have been proposed to serve as
catalysts for H$_2$-formation. Our results indicate that carbon backbone
fragmentation may be a serious competitor to H$_2$-formation at least for small
hydrogenated PAHs like pyrene.
