Broadband rotational spectroscopy in Laval flows: applications in astrochemistry and dynamics
Authors:   , Dias Nureshan 
Published: February 2023

[EMBARGOED UNTIL 8/1/2023] This dissertation presents the successful implementation of a Chirped pulse Uniform Flow (CPUF) apparatus capable of conducting experiments at low temperatures to examine photodissociation/reaction dynamics and kinetics studies relevant to astronomical environments. In CPUF approach, a pulsed uniform flow (which is equivalent to a wall-less reactor at constant temperature and pressure) produced by expansion of a gas through a Laval nozzle is combined with broadband mm-wave detection. The CPUF technique has provided unambiguous, universal detection of any molecules or transient species with a modest dipole moment thermalized to the uniform temperature of the gas flow, with isomer, conformer, and vibrational level specificity. Chirped-pulse microwave detection requires monitoring the free induction decay on the timescale of microseconds, which limits sensitivity at higher densities. This dissertation discusses two key approaches used to address these challenges: In one approach, we use a modified "quasi-uniform" flow where the initial conditions are uniform and high density. This is followed by a second expansion to a very cold, low-density region optimal for the mm-wave detection. Aided by detailed characterization of this flow condition by experiments and fluid dynamic simulations, product branching as well as observation of product evolution as a function of time can be measured. Several photodissociation and bimolecular reaction studies relevant to planetary atmospheres and interstellar medium were investigated employing quasi uniform flows. The photodissociation of Isoxazole, and the propargyl radical will be discussed. Guided by previous electronic structure and dynamics simulations, we were able to elucidate the dissociation dynamics that govern the final product branching fractions observed in these dissociations. Additionally, radical-radical reaction dynamics of NH2/ND2+ C3H3 and NO + C3H3 were studied employing quasi uniform flows. These reactions showcase the power of CPUF ability on multichannel branching measurements for a complex radical-radical reaction. We then present a study of the direct D-H exchange reaction in radicals, demonstrating that it is an important and overlooked pathway for deuterium fractionation in astrochemical environments. For application to low-temperature kinetics studies, a truly uniform flow is required to obtain reliable rate measurements and enjoy all the advantages that CP-FTMW has to offer. To this end we present a new setup that combines sampling of uniform supersonic flows using an airfoil-shaped sampling device with chirped-pulse mmW detection. Density and temperature variations in the airfoil-sampled uniform flow were revealed using time-dependent rotational spectroscopy of pyridine photoproducts, highlighting the use of UV photodissociation as a sensitive diagnostic tool for uniform flows. We have also made the first measurement of reaction kinetics in a low-temperature uniform flow using mmW detection, by sampling the flow to lower density and temperature. This combination shows considerable promise for studying low temperature kinetics related to astrochemical environments.