UF-CRDS: A PULSED UNIFORM FLOW APPARATUS WITH CW-CAVITY-RINGDOWN SPECTROSCOPY

We introduce a new apparatus in which a high-performance pulsed supersonic uniform flow from a Laval nozzle is coupled with a continuous wave cavity ringdown spectroscopy (cw-CRDS). This approach is related to the CRESU\footnote{Sims,I., Queffelec, J.L., Defrance,A., Rebrion-Rowe,C., Travers,D., Bocherel, P., Rowe, B., and Smith, I.W.M., J. Chem. Phys. 100 (6), 4229-4241 (1994).} technique developed in France to study reaction kinetics at low temperature. A related system developed in our group in which chirped-pulse microwave spectroscopy is coupled to a pulsed Laval flow has successfully demonstrated its investigative capability of isomer-specific product branching in reactions and photodissociation at temperatures as low as 22 K\footnote{Broderick, B.M., Suas-David, N., Dias, N., and Suits, A.G., Phys. Chem. Chem. Phys. 20 (8), 5517-5529 (2018).}. The pulsed uniform flow is produced by means of a high throughput piezoelectric stack valve combined with a Laval nozzle.\footnote{Oldham, J.M., Abeysekera, C., Joalland, B., Zack, L.N., Prozument, K., Sims, I.R., Park, G.B., Field, R.W., and Suits, A.G., J. Chem. Phys. 141 (15), 154202 (2014).}At present, we employ two machined aluminum nozzles (for carrier gases He and Ar at temperatures around 25 K), and numerous in-house 3D printed nozzles. The 3D printed nozzles are designed using a Matlab program developed in-house, which allows us to create supersonic uniforms flows with different carrier gases at various temperature and densities. These nozzles are validated experimentally as well as theoretically using a computational fluid dynamics program, OpenFOAM. The current configuration can probe the pulsed uniform flow either by cw-CRDS, operated in the near infrared region, or laser-induced fluorescence as in the traditional CRESU approach. The cw-CRDS spectrometer consists of a high finesse optical cavity (F~200000) which is composed of two high reflective plano-concave mirrors (R~99.9988\%) leading to an empty cavity decay constant of ~160 $\mu$s. We adopt a modified version of the timing strategy which was reported by Hippler M. et al\footnote{Hippler, M., and Quack, M., M Hippler, et.al, Chem. Phys. Lett. 314 (3), 273-281 (1999).} in order to probe reactants of bimolecular reactions formed from photolysis. We will present our first low temperature kinetics experiments performed with this apparatus including reaction of CN (v=1) with alkenes probed by cw-CRDS