Journal Article

PUBDB-2023-00931

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Cooling dynamics of energized naphthalene and azulene radical cations

Lee, J. W. L.Extern*DESY* ; Stockett, M. H. ; Ashworth, E. K. ; Navarro Navarrete, J. E. ; Gougoula, E.DESY* ; Garg, D.DESY* ; Ji, M. ; Zhu, B. ; Indrajith, S. ; Zettergren, H. ; Schmidt, H. T. ; Bull, J. N. (Corresponding author)

2023
American Institute of Physics Melville, NY

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Please use a persistent id in citations: doi:10.1063/5.0147456  doi:10.3204/PUBDB-2023-00931

Abstract: 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 C$_2$H$_2$-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$^+$ D$_0$ ← D$_2$ 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.

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Contributing Institute(s):

1. Spectroscopy of molecular processes (FS-SMP)

Research Program(s):

1. 631 - Matter – Dynamics, Mechanisms and Control (POF4-631) (POF4-631)

Experiment(s):

1. Measurement at external facility

Appears in the scientific report 2023

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Database coverage:
; ; Clarivate Analytics Master Journal List ; Current Contents - Physical, Chemical and Earth Sciences ; Ebsco Academic Search ; Essential Science Indicators ; IF < 5 ; JCR ; National-Konsortium ; PubMed Central ; SCOPUS ; Science Citation Index Expanded ; Web of Science Core Collection

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