Design and Implementation of a New Apparatus for Astrochemistry: Kinetic Measurements of the CH + OCS Reaction and Frequency Comb Spectroscopy in a Cold Uniform Supersonic Flow

Abstract

We present the development of a new astrochemical research tool HILTRAC, the Highly Instrumented Low Temperature ReAction Chamber. The instrument is based on a pulsed form of the CRESU (Cin\'etique de R\'eaction en \'Ecoulement Supersonique Uniforme, meaning reaction kinetics in a uniform supersonic flow) apparatus, with the aim of collecting kinetics and spectroscopic information on gas phase chemical reactions important in interstellar space or planetary atmospheres. We discuss the apparatus design and its flexibility, the implementation of pulsed laser photolysis followed by laser induced fluorescence (PLP-LIF), and the first implementation of direct infrared frequency comb spectroscopy (DFCS) coupled to the uniform supersonic flow. Achievable flow temperatures range from 32(3) - 111(9) K, characterising a total of five Laval nozzles for use with N2 and Ar buffer gases by pressure impact measurements. These results were further validated using LIF and DFCS measurements of the CH radical and OCS, respectively. Spectroscopic constants and linelists for OCS are reported for the 1001 band near $2890 - 2940 cm^{-1}$ for both $OC^{32}S$ and $OC^{34}S$, measured using DFCS. Additional peaks in the spectrum are tentatively assigned to the OCS-Ar complex. The first reaction rate coefficients for the CH + OCS reaction measured between 32(3) K and 58(5) K are reported. The reaction rate coefficient at 32(3) K was measured to be $3.9(4) \times 10^{10} cm^3 molecule^{-1} s^{-1}$ and the reaction was found to exhibit no observable temperature dependence over this low temperature range.