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Laboratory Kinetic Measurements to Unravel Cosmic Carbon Chemistry

Presentation #122.02 in the session “Laboratory Astrophysics Division (LAD): Sample Return & Aromatic/Exotic Molecules in Space”.

Published onJun 18, 2021
Laboratory Kinetic Measurements to Unravel Cosmic Carbon Chemistry

Low-temperature reactions between CN and aromatic molecules are of significant interest in the astrochemical community due to the recent detection of CN-tagged aromatics in the interstellar medium (ISM), including the first detection of polycyclic aromatic hydrocarbons (PAHs). Pure PAHs are challenging to detect due to their overlapping infrared bands as well as their low or zero dipole moments. It has therefore been suggested that the CN-substituted analogs (e.g. benzonitrile, cyano-naphthalene) can be used as chemical proxies to establish the abundances of their pure aromatic counterparts. Laboratory kinetics measurements at low temperatures are thus critical to determine whether CN-substituted aromatics can be used as robust observational proxies. We have studied the reactions 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 are temperature independent 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. Virtually no product branching ratios have been measured experimentally at the low temperatures relevant to dense interstellar clouds. I will discuss our recent progress in combining chirped-pulsed micro/mm-wave spectroscopy with the CRESU technique in order to measure low-temperature product branching ratios.

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