Low temperature reaction kinetics in a uniform flow inside an extended Laval nozzle characterized by REMPI and probed by rotational spectroscopy Shameemah Thawoos 1 , Nicolas Suas-David 2 , Matthew Edlin 1 , Ranil Gurusinghe 1,3 and Arthur G. Suits 1 1 Department of Chemistry, University of Missouri, Columbia, MO, 65211 USA 2 Univ Rennes, CNRS, Institut de Physique de Rennes - UMR 6251, F-35000 Rennes, France 3 Department of Chemistry, Tennessee Technical University, Cookeville, TN 38505 USA Chirped-Pulse Fourier-Transform Microwave (CP-FTMW) spectroscopy is a groundbreaking, near-universal detection method which enables rapid “fingerprint” detection of quantum state specific reactants and products within mixtures. We have successfully coupled this method with a pulsed CRESU system to study reaction kinetics at very low temperature, which we refer to as CPUF (“Chirped-Pulse/Uniform flow”). CRESU is a French acronym for reaction kinetics in uniform supersonic flow. The uniform supersonic flow is generated by expanding a gas from stagnation to a low-pressure region through a Laval nozzle. The traditional method of detection coupled with CRESU laser induced fluorescence (LIF). One of the major drawbacks of LIF is the limited number of species that can be probed, and this range can be greatly expanded with the CPUF method. Detection by CPUF requires monitoring the free induction decay of the species. However, the high collisional environment in high-density uniform supersonic flows can interfere with the free induction decay and attenuate the signal. One way to overcome this is to use sampling methods, such as airfoil or skimmer sampling. However, this requires complicated experimental designs with differential pumping systems and can have interference from shocks in the sampling region and thus may not be ideal for studying reaction kinetics. This has led us to develop an extended Laval nozzle which creates a uniform flow within the nozzle itself, after which the gas undergoes a shock-free secondary expansion to a cold, low pressure condition ideal for CPUF detection. However, impact pressure measurements, commonly used to characterize Laval flows, cannot be used to monitor the flow within the nozzle. Hence, we have implemented a REMPI (resonance-enhanced multiphoton ionization) detection scheme which allows interrogation of the conditions of the flow directly inside the extended nozzle, confirming fluid dynamics simulations of the flow environment. We have built an extended nozzle designed for a 22 K He flow and characterized the flow within the nozzle using (1+1) REMPI of a very dilute sample of NO. We will describe the development of the new extended flow along with its characterization using REMPI and initial application to studies of the low temperature reaction kinetics of HCO with NO and O 2 as co-reactants.
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