Survival of the brightest: radiative stabilization of interstellar polycyclic aromatic hydrocarbons

After decades of speculation and inconclusive searches, radio astronomers have begun to identify specific polycyclic aromatic hydrocarbon (PAH) molecules in space. Beginning in 2021, two isomers of cyano-naphthalene C₁₀H₇CN, indene C₉H₈, and 2-cyano-indene C₉H₇CN were detected in the Taurus Molecular Cloud (TMC-1) by comparing astronomical microwave spectra with known rotational emission lines. Very recently, several larger cyano-substituted PAHs have been identified using the same methods. These observations have challenge state-of-the-art astrochemical models, which underpredict the observed abundances small PAHs by several orders of magnitude. Potential contributions to these discrepancies are overestimation of the destruction rates of PAHs in the models, and the underestimation of the abundance of their precursors such as naphthalene C₁₀H₈.

Using the DESIREE cryogenic electrostatic ion-beam storage ring, we have investigated the stability against unimolecular dissociation of the vibrationally hot cations of 1-cyano-naphthalene, naphthalene and its isomer azulene, indene and its primary degradation product indenyl C₉H₇, and 2-cyano-indene. These measurements reveal the rates and mechanisms of radiative cooling of these cations, which are key intermediates in the destruction pathways posited by astrochemical models. Most of these ions are radiatively stabilized by Recurrent Fluorescence (RF), the emission of optical photons from thermally excited electronic states. Radiative stabilization by RF closes some of the destruction channels included in astrochemical models, helping to rationalize their underprediction of observed abundances. Consideration of symmetry breaking and vibronic coupling are essential to explaining the measured RF rates. Measurements of the distributions of kinetic energy released in the dissociation provide detailed information on the reactions’ potential energy surfaces.