Dissociation and radiative stabilization of an interstellar Polycyclic Aromatic Hydrocarbon cation

In 2021, after decades of inconclusive searches, astronomers identified the first specific Polycyclic Aromatic Hydrocarbon (PAH) molecules in space. Using the Greenbank Radio Telescope, McGuire et al. [Science 371, 1265 (2021)] identified two isomers of cyanonaphthalene (CNN) in the Taurus Molecular Cloud (TMC-1). State-of-the-art astrochemical modeling underpredicts the observed abundance of CNN and other small PAHs identified in TMC-1 by 4-6 orders of magnitude.

In a pair of recent reports, we address this gap by elucidating the main destruction pathway for CNN. Using the cryogenic electrostatic ion-beam storage ring DESIREE, we determine the absolute unimolecular dissociation rate coefficient from the time dependent neutral yield and Kinetic Energy Release distributions of stored beams of vibrationally hot 1-cyanonaphthalene (1-CNN) cations. We find that, contrary to the explicit assumption of McGuire et al., 1-CNN is efficiently stabilized by rapid radiative cooling following ionization, closing off some of the reaction channels assumed to deplete CNN from TMC-1. Crucially, vibronic coupling greatly increases the radiative relaxation rate of 1-CNN⁺ by enhancing Recurrent Fluorescence, the emission of optical photons from thermally excited electronic states.