Recurrent Fluorescence of Polycyclic Aromatic Hydrocarbon Isomers: A Comparative Study

Time-dependent unimolecular dissociation rates of the C₁₂H₈ isomers acenaphthylene (ACY) and biphenylene (BPY) cations were measured using a cryogenic electrostatic ion beam storage ring. The neutral, cyano-functionalized tracers of ACY, but not of BPY, have been identified in the interstellar molecular cloud TMC-1 by radioastronomy. For both polycyclic aromatic hydrocarbons (PAHs), dissociation is rapidly quenched by recurrent fluorescence (RF). Master equation simulations including RF rate coefficients based on <i>ab initio</i> molecular dynamics calculations reproduce the measured dissociation rates. Only marginal differences in the survival probabilities of ACY and BPY in TMC-1 are indicated by these results, with both cations being stable for vibrational energies up to about 7.6 eV, which is 3 eV above the dissociation threshold energy.