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.