Laboratory measurements of gas-phase reactions between CN and aromatic molecules at low temperatures

Low-temperature reactions between CN and aromatic molecules 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. This detection has caused excitement due to the potential link between small aromatic molecules and polycyclic aromatic hydrocarbons (PAHs). It remains an open question as to whether PAHs can be efficiently formed in the ISM by bottom-up mechanisms involving growth from small aromatic precursors like benzene. Benzonitrile is likely formed in dense clouds via the neutral-neutral gas-phase reaction between CN and benzene. It has therefore been suggested that benzonitrile can be used as a chemical proxy to establish the abundance of benzene, which itself cannot be detected with radio telescopes due to its lack of a permanent dipole moment. Gas-phase reactions between other aromatic molecules and CN are likewise expected to be rapid at low temperature and may facilitate the detection of other CN-substituted carbon rings and PAHs. However, laboratory kinetics measurements are required to determine whether CN-substituted aromatics can be used as robust observational proxies. Both rate coefficients and product-branching ratios for the reactions between CN and small aromatic molecules must be measured at low temperature in order to input these reactions into astrochemical models and accurately predict abundances. We have studied the reaction of the CN radical with benzene and with toluene, from room temperature down to 15 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 have found that the rate coefficients do not display obvious temperature dependences between 15 K and room temperature, confirming that these reactions will remain rapid at temperatures relevant to the cold ISM. While CRESU PLP-LIF measurements can be used to measure rate coefficients for the overall reactions, the product-channel-specific reaction rate coefficients cannot be measured using this technique. Only a handful of product branching ratios have been measured experimentally at the low temperatures relevant to dense interstellar clouds. This is in part due to the difficulty of developing a detection technique which is not only highly sensitive and specific but also able to quantitatively measure the concentrations of several reaction products at the same time. I will discuss our recent progress in combining chirped-pulsed micro/mm-wave spectroscopy with the CRESU technique in order to measure the low-temperature product branching ratios.