Cooling dynamics of energized naphthalene and azulene radical cations

Lee, Jason W. L., Stockett, Mark H., Ashworth, Eleanor K., Navarro Navarrete, José E., Gougoula, Eva, Garg, Diksha, Ji, MingChao, Zhu, Boxing, Indrajith, Suvasthika, Zettergren, Henning, Schmidt, Henning T. and Bull, James N. ORCID: (2023) Cooling dynamics of energized naphthalene and azulene radical cations. The Journal of Chemical Physics, 158 (17). ISSN 0021-9606

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Naphthalene and azulene are isomeric polycyclic aromatic hydrocarbons (PAHs) and are topical in the context of astrochemistry due to the recent discovery of substituted naphthalenes in the Taurus Molecular Cloud-1 (TMC-1). Here, the thermal- and photo-induced isomerization, dissociation, and radiative cooling dynamics of energized (vibrationally hot) naphthalene (Np+) and azulene (Az+) radical cations, occurring over the microsecond to seconds timescale, are investigated using a cryogenic electrostatic ion storage ring, affording “molecular cloud in a box” conditions. Measurement of the cooling dynamics and kinetic energy release distributions for neutrals formed through dissociation, until several seconds after hot ion formation, are consistent with the establishment of a rapid (sub-microsecond) Np+ ⇌ Az+ quasi-equilibrium. Consequently, dissociation by C2H2-elimination proceeds predominantly through common Az+ decomposition pathways. Simulation of the isomerization, dissociation, recurrent fluorescence, and infrared cooling dynamics using a coupled master equation combined with high-level potential energy surface calculations [CCSD(T)/cc-pVTZ], reproduce the trends in the measurements. The data show that radiative cooling via recurrent fluorescence, predominately through the Np+ D0 ← D2 transition, efficiently quenches dissociation for vibrational energies up to ≈1 eV above dissociation thresholds. Our measurements support the suggestion that small cations, such as naphthalene, may be more abundant in space than previously thought. The strategy presented in this work could be extended to fingerprint the cooling dynamics of other PAH ions for which isomerization is predicted to precede dissociation.

Item Type: Article
Uncontrolled Keywords: physics and astronomy(all),physical and theoretical chemistry ,/dk/atira/pure/subjectarea/asjc/3100
Faculty \ School: Faculty of Science > School of Chemistry
UEA Research Groups: Faculty of Science > Research Groups > Centre for Photonics and Quantum Science
Faculty of Science > Research Groups > Chemistry of Light and Energy
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Depositing User: LivePure Connector
Date Deposited: 09 May 2023 08:30
Last Modified: 15 May 2023 08:31
DOI: 10.1063/5.0147456


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