  After decades of speculation and searching, astronomers have recently
identified specific Polycyclic Aromatic Hydrocarbons (PAHs) in space.
Remarkably, the observed abundance of cyanonaphthalene (CNN, C10H7CN) in the
Taurus Molecular Cloud (TMC-1) is six orders of magnitude higher than expected
from astrophysical modeling. Here, we report absolute unimolecular dissociation
and radiative cooling rate coefficients of the 1-CNN isomer in its cationic
form. These results are based on measurements of the time-dependent neutral
product emission rate and Kinetic Energy Release distributions produced from an
ensemble of internally excited 1-CNN + studied in an environment similar to
that in interstellar clouds. We find that Recurrent Fluorescence - radiative
relaxation via thermally populated electronic excited states - efficiently
stabilizes 1-CNN+ , owing to a large enhancement of the electronic transition
probability by vibronic coupling. Our results help explain the anomalous
abundance of CNN in TMC-1 and challenge the widely accepted picture of rapid
destruction of small PAHs in space.
