Superhydrogenated PAHs: Catalytic formation of H 2

The possible role of neutral PAHs as catalysts for H-2 formation in the interstellar medium is investigated by a combined experimental and density function theory study of the superhydrogenation of coronene (C24H12). The calculations suggest efficient hydrogenation of both edge and centre sites, along with competing abstraction reactions to form H-2 in a series of catalytic cycles. Scanning tunneling microscopy and thermal desorption measurements have been used to provide direct evidence of the formation of superhydrogenated coronene as a result of exposure to D atoms. Lower limit estimates for the cross-sections of 1.8 x 10(-17), 5.5 x 10(-18) and 1.1 x 10(-18) cm(2) for the formation of singly, doubly and triply hydrogenated coronene are derived. The results suggest that superhydrogenated PAHs may play an important role in H-2 formation in the ISM.