Cooling dynamics of energized naphthalene and azulene radical cations.

Autor:	Lee JWL; Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany., Stockett MH; Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden., Ashworth EK; School of Chemistry, Norwich Research Park, University of East Anglia, Norwich NR4 7TJ, United Kingdom., Navarro Navarrete JE; Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden., Gougoula E; Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany., Garg D; Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany., Ji M; Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden., Zhu B; Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden., Indrajith S; Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden., Zettergren H; Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden., Schmidt HT; Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden., Bull JN; School of Chemistry, Norwich Research Park, University of East Anglia, Norwich NR4 7TJ, United Kingdom.
Jazyk:	angličtina
Zdroj:	The Journal of chemical physics [J Chem Phys] 2023 May 07; Vol. 158 (17).
DOI:	10.1063/5.0147456
Abstrakt:	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. (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)
Databáze:	MEDLINE
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