Dissociation and radiative stabilization of the indene cation: The nature of the C-H bond and astrochemical implications

Indene (C$_9$H$_8$) is the only polycyclic pure hydrocarbon identified in the interstellar medium to date, with an observed abundance orders of magnitude higher than predicted by astrochemical models. The dissociation and radiative stabilization of vibrationally-hot indene cations is investigated by measuring the time-dependent neutral particle emission rate from ions in a cryogenic ion-beam storage ring for up to 100~ms. Time-resolved measurements of the kinetic energy released upon hydrogen atom loss from C$_9$H$_8^+$, analyzed in view of a model of tunneling through a potential energy barrier, provides the dissociation rate coefficient. Master equation simulations of the dissociation in competition with vibrational and electronic radiative cooling reproduce the measured dissociation rate. We find that radiative stabilization arrests one of the main C$_9$H$_8$ destruction channels included in astrochemical models, helping to rationalize its high observed abundance.