Unimolecular fragmentation and radiative cooling of isolated PAH ions: A quantitative study

Abstract

Time-resolved spontaneous and laser-induced unimolecular fragmentation of perylene cations (C 20 H 12 + ) has been measured on timescales up to 2 s in a cryogenic electrostatic ion beam storage ring. We elaborate a quantitative model, which includes fragmentation in competition with radiative cooling via both vibrational and electronic (recurrent fluorescence) de-excitation. Excellent agreement with experimental results is found when sequential fragmentation of daughter ions co-stored with the parent perylene ions is included in the model. Based on the comparison of the model to experiment, we constrain the oscillator strength of the D 1 → D 0 emissive electronic transition in perylene (f RF = 0.055 ± 0.011), as well as the absolute absorption cross section of the D5 ← D 0 excitation transition (σ abs > 670 Mb). The former transition is responsible for the laser-induced and recurrent fluorescence of perylene, and the latter is the most prominent in the absorption spectrum. The vibrational cooling rate is found to be consistent with the simple harmonic cascade approximation. Quantitative experimental benchmarks of unimolecular processes in polycyclic aromatic hydrocarbon ions like perylene are important for refining astrochemical models.

Radiative cooling rates of substituted PAH ions

“…The decay curve analysis procedure could be applied to laser-induced decay measurements to track the evolution of the vibrational energy distribution on longer timescales 42,43 . It could also be applied in more complex situations where multiple species are present in the stored ion beam 12 or where competing channels yield decay curves that are challenging to model analytically 33,37 .…”

“…The time evolution of the population distribution g(E, t) of intact OHPyr + and BrPyr + were simulated using the Master Equation approach 28 . Further details of the methodology are presented elsewhere 12 . The vibrational level density ρ(E) is computed using the the Beyer-Swinehart algorithm 29 .…”

“…7. In a previous report 12 , det was determined by comparing the neutral yield at long times, after the spontaneous decay rate reaches the floor set by the residual gas collision rate, with the beam storage lifetime determined by measuring N stored as a function of storage time. In the present experiments, however, the ion current at the end of the storage cycle was too low to be measured with the Faraday cup, due to the need to limit saturation of the detector by high count rates.…”

“…The survival of PAHs in harsh interstellar environments depends on the interplay between photo-and collision-induced dissociation and radiative stabilization 9 . Electrostatic Storage Devices (ESDs) allow highly-excited PAH ions to be isolated in collision-free environments for timescales up to tens of seconds, enabling determination of radiative cooling rates over several orders of magnitude in time [10][11][12] . An important finding of such studies has been the significance of recurrent fluorescence in stabilizing hot PAH cations 13,14 .…”

Thermal radiative cooling of carbon cluster cations C$_N^+$, $N = 9, 11,12, 17-27$

“…[6][7][8][9][10][11] for experiments on carbon anions and Refs. [10,[12][13][14][15] for PAH molecules and other carbon-containing ions. Studies of radiative cooling of cationic carbon clusters were reported in [16].…”