Benzonitrile as a proxy for benzene in the cold ISM: low temperature
  rate coefficients for CN + C$_6$H$_6$
  The low temperature reaction between CN and benzene (C$_6$H$_6$) is of
significant interest in the astrochemical community due to the recent detection
of benzonitrile, the first aromatic molecule identified in the interstellar
medium (ISM) using radio astronomy. Benzonitrile is suggested to be a low
temperature proxy for benzene, one of the simplest aromatic molecules, which
may be a precursor to polycyclic aromatic hydrocarbons (PAHs). In order to
assess the robustness of benzonitrile as a proxy for benzene, low temperature
kinetics measurements are required to confirm whether the reaction remains
rapid at the low gas temperatures found in cold dense clouds. Here, we study
the C$_6$H$_6$ + CN reaction in the temperature range 15--295 K, using the
well-established CRESU technique (a French acronym standing for Reaction
Kinetics in Uniform Supersonic Flow) combined with Pulsed Laser
Photolysis-Laser-Induced Fluorescence (PLP-LIF). We obtain rate coefficients,
$k(T)$, in the range (3.6--5.4) $\times$ 10$^{-10}$ cm$^3$ s$^{-1}$ with no
obvious temperature dependence between 15--295 K, confirming that the CN +
C$_6$H$_6$ reaction remains rapid at temperatures relevant to the cold ISM.
