PHOTON-INDUCED FORMATION OF MOLECULAR HYDROGEN FROM A NEUTRAL POLYCYCLIC AROMATIC HYDROCARBON: 9,10-DIHYDROANTHRACENE

Experimental results from infrared spectroscopy and mass spectrometry provide compelling evidence that UV irradiation of the neutral polycyclic aromatic hydrocarbon (PAH) 9,10-dihydroanthracene (DHA), trapped in solid argon (12 K), results in efficient (i.e., 90% yield) conversion to anthracene and molecular hydrogen. A number of dissociation pathways involving single or double hydrogen loss are investigated computationally. Among these, two mechanisms are most credible for a one-photon dissociation process involving UV photons <5.5 eV. For the lowest-energy pathway (2.3 eV), a simultaneous lengthening of the C-H bonds of H9 and H10 gives rise to an anthracene-H-2 complex. A higher-energy mechanism (3.4 eV) involves an initial lengthening of the H9 C-H bond, followed by this hydrogen "grabbing" H10, and forming H-2. The high yield of this photolysis reaction suggests that similar reactions may take place for other neutral PAHs, with implications for the formation of molecular hydrogen in regions of low UV exposure, such as in dark clouds.