Cooling dynamics of energized naphthalene and azulene radical cations

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 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.

Fragmentation of Multiply Charged C10H8 Isomers Produced in keV Range Proton Collision

“…Overall, there is little difference between the fragmentation mass spectra of az and nph in high-energy collisions, which may be due to the lower energy difference between multiply charged isomers compared to singly charged or neutral isomers. In a recent paper, Lee et al [26] demonstrated that even at much lower internal energies than in the present study, an equilibrium can be established between nph + and az + before dissociation. In fact, the energy transfer to the molecule is expected to be very high (typically few tens of eV) in collisional excitation with fast proton, so that the intact molecular ion can explore all available isomers prior to dissociation, independently from the initially chosen isomer.…”

In search of universalities in the dissociative photoionization of PANHs via isomerizations

“…31 and 32) prior to acetylene abstraction, is a seven member ring containing the isomer. This was recently reaffirmed independently by another group wherein it was concluded via analysis of kinetic energy release distributions measurements 54 that the ions accompanying acetylene abstraction was majorly pentalene with another minor product being phenylacetylene. Incidentally, these are the same products that are suggested to form after HCN abstraction from quinoline too.…”

Protomers of the green and cyan fluorescent protein chromophores investigated using action spectroscopy

“…There were no obvious marker m / z products from a given protomer ensemble that clearly reflect the protonation site, presumably because dissociation is statistical in nature and rearrangement barriers are lower in energy than dissociation barriers. 39…”