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We have investigated the photo-stability of pristine and super-hydrogenatedpyrene cations C$_{16}$H$_{10+m}^+, m = 0,6, \mathrm{\ or\ } 16$) by means ofgas-phase action spectroscopy. Optical absorption spectra and photo-induceddissociation mass spectra are presented. By measuring the yield ofmass-selected photo-fragment ions as a function of laser pulse intensity, thenumber of photons (and hence the energy) needed for fragmentation of the carbonbackbone was determined. Backbone fragmentation of pristine pyrene ions(C$_{16}$H$_{10}^+$) requires absorption of three photons of energy just below3 eV, whereas super-hydrogenated hexahydropyrene (C$_{16}$H$_{16}^+$) mustabsorb two such photons and fully hydrogenated hexadecahydropyrene(C$_{16}$H$_{26}^+$) only a single photon. These results are consistent withpreviously reported dissociation energies for these ions. Our experimentsclearly demonstrate that the increased heat capacity from the additionalhydrogen atoms does not compensate for the weakening of the carbon backbonewhen pyrene is hydrogenated. In photodissociation regions, super-hydrogenatedPolycyclic Aromatic Hydrocarbons (PAHs) have been proposed to serve ascatalysts for H$_2$-formation. Our results indicate that carbon backbonefragmentation may be a serious competitor to H$_2$-formation at least for smallhydrogenated PAHs like pyrene.

Photo-stability of super-hydrogenated PAHs determined by action spectroscopy experiments