Recurrent Fluorescence of Polycyclic Aromatic Hydrocarbon Isomers: A Comparative Study

Area of Science:

• Astrochemistry
• Physical Chemistry
• Computational Chemistry

Background:

• Polycyclic aromatic hydrocarbons (PAHs) are abundant in interstellar environments.
• Radioastronomy has detected cyano-functionalized tracers of acenaphthylene (ACY) but not biphenylene (BPY) in the TMC-1 molecular cloud.
• Understanding the stability of PAH cations is crucial for astrochemistry.

Purpose of the Study:

• To measure the time-dependent unimolecular dissociation rates of ACY and BPY cations.
• To assess the survival probabilities of ACY and BPY cations in the interstellar medium (ISM).
• To investigate the role of recurrent fluorescence (RF) in quenching dissociation.

Main Methods:

• Utilized a cryogenic electrostatic ion beam storage ring to measure dissociation rates.
• Employed master equation simulations incorporating ab initio molecular dynamics calculations for RF rate coefficients.
• Compared experimental data with theoretical simulations.

Main Results:

• Dissociation for both ACY and BPY cations is significantly quenched by recurrent fluorescence (RF).
• Master equation simulations accurately reproduced the measured dissociation rates.
• Both ACY and BPY cations exhibit marginal differences in survival probabilities in TMC-1.
• Cations remain stable for vibrational energies up to 7.6 eV, well above their dissociation thresholds.

Conclusions:

• ACY and BPY cations are likely stable in the interstellar molecular cloud TMC-1.
• Recurrent fluorescence plays a critical role in stabilizing these PAH cations.
• The observed radioastronomical detection of ACY tracers, but not BPY, may not be solely due to intrinsic cation stability differences.