Photo-stability of super-hydrogenated PAHs determined by action spectroscopy experiments
- Department of Physics, Stockholm University, SE-106 91 Stockholm (Sweden)
We have investigated the photo-stability of pristine and super-hydrogenated pyrene cations (C{sub 16}H{sub 10+m}{sup +},m=0,6, 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{sub 16}H{sub 10}{sup +}) requires absorption of three photons of energy just below 3 eV, whereas super-hydrogenated hexahydropyrene (C{sub 16}H{sub 16}{sup +}) must absorb two such photons and fully hydrogenated hexadecahydropyrene (C{sub 16}H{sub 26}{sup +}) 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{sub 2} formation. Our results indicate that carbon backbone fragmentation may be a serious competitor to H{sub 2} formation at least for small hydrogenated PAHs like pyrene.
- OSTI ID:
- 22868461
- Journal Information:
- Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 832; ISSN ASJOAB; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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