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 ab initio 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.