CONTINUOUS-WAVE CAVITY RING-DOWN SPECTROSCOPY IN A PULSED UNIFORM SUPERSONIC FLOW

\begin{wrapfigure}{r}{0pt}_x000d_ \includegraphics[scale=0.2]{UF-CRDS.eps}_x000d_ \end{wrapfigure}_x000d_ _x000d_ _x000d_ _x000d_ We introduce a new approach that couples a pulsed uniform supersonic flow with high sensitivity continuous wave cavity ringdown spectroscopy (UF-CRDS) operated in the near infrared (NIR). _x000d_ _x000d_ This combination is related to the CRESU\footnote{I. Sims, J. L. Queffelec, A. Defrance, C. Rebrion-Rowe, D. Travers, P. Bocherel, B. Rowe, I. W. Smith, J. Chem. Phys. 100, 4229-4241, (1994).} technique developed in France and used for many years to study reaction kinetics at low temperature, and to the microwave based chirped-pulse uniform supersonic flow spectrometer (CPUF) developed in our group which has successfully demonstrated the use of pulsed uniform supersonic flow to probe reaction dynamics at temperatures as low as 22 K\footnote{C. Abeysekera, B. Joalland, N. Ariyasingha, L. N. Zack, I. R. Sims, R. W. Field, A. G. Suits, J. Phys. Chem. Lett. 6, 1599-1604, (2015).}. CRDS\footnote{N. Suas-David, T. Vanfleteren, T. F\"{o}ldes, S. Kassi, R. Georges, M. Herman, J. Phys. Chem.A, 119, 10022-10034, (2015).} operated with NIR permits access to the first overtones of C-H and O-H stretching/bending which, in combination with its extraordinary sensitivity opens new experiments complementary to the CPUF technique._x000d_ The UF-CRDS apparatus (Figure) utilizes the pulsed uniform flow produced by means of a piezo-electric stack valve\footnote{C. Abeysekera, B. Joalland, Y. Shi, A. Kamasah, J. M. Oldham, A. G. Suits, Rev. Sci. Instrum. 85, 116107, (2014).} in combination with a Laval nozzle. At present, two machined aluminum Laval nozzles designed for carrier gases Ar and He generate flows with a temperature of approximately 25 K and pressure around 0.15 mbar. This flow is probed by an external cavity diode laser in the NIR (1280-1380 nm). Laval nozzles designed using a newly developed MATLAB-based program will be used in the future. A detailed illustration of the novel UF-CRDS instrumentation and its performance will be presented along with future directions and applications. _x000d_ _x000d_ _x000d