Photon-induced complete dehydrogenation of putative interstellar polycyclic aromatic hydrocarbon cations: Coronene and naphtho[2,3-a]pyrene

Fourier transform ion cyclotron resonance mass spectra of coronene and naphtho[2, 3-a]pyrene radical cations exposed to broadband ultraviolet/visible radiation are presented. Parent ions are formed and confined in a Penning trap at 3.0 T and photolyzed with an Xe are lamp. Each of the two radical cations is found to dehydrogenate completely to leave the C-24(+) bare carbon cluster cation. The extent of dehydrogenation depends directly on irradiation power. We infer that polycyclic aromatic hydrocarbon cations are inherently unstable to photolysis. We discuss some implications of hydrogen loss to yield bare carbon cluster cations. The relevance of dehydrogenation to unidentified interstellar infrared emission bands is discussed briefly.