UF-CRDS: a pulsed uniform supersonic flow apparatus coupled with continuous-wave cavity ringdown spectroscopy
Authors:   , Thawoos Shameemah 
Published: September 2022

Investigating low temperature reaction kinetics of elementary reactions is key to understanding many phenomena of astrochemical importance. Since the introduction of CRESU (A French acronym that stands for reactions kinetics in uniform supersonic flow) method it has been widely used to study many reactions at temperatures as low as 13 K. The uniform supersonic flow provides a continuous flow with unform temperature and density which act as a wall-less configuration avoiding condensations effects along the walls when cooling down to low temperatures. The uniform flow is achieved by a Laval nozzle (a convergent-divergent nozzle). Traditionally, laser induced fluorescence (LIF) is used to probe reactants in the flow. In this thesis I describe a new instrument in which highly sensitive continuous wave-cavity ringdown spectroscopy (cw-CRDS) is coupled with a pulsed uniform flow for the first time, a "uniform flow cavity ringdown spectrometer," UF-CRDS. The UF-CRDS setup is equipped with a pulsed uniform flow system which is produced by means of a high throughput piezoelectric stack valve combined with a Laval nozzle. In addition to the Laval nozzles built in collaboration with Dr. I.R. Sims from University of Renne, 3D printed Laval nozzles designed using a Matlab program developed in-house are also used. These nozzles are validated experimentally as well as theoretically using a computational fluid dynamics program, OpenFOAM. Cavity ringdown spectroscopy is an absorption technique which measures the rate of decay of trapped light with an optical cavity made by two high reflectivity mirrors. As this method measures the rate of decay of light instead of light intensity, it is largely immune the intensity fluctuations of the light source. The UF-CRDS apparatus is equipped with a DFB diode laser or an ECDL laser that can be tuned between 1411-1419 nm and 1280-1380 nm respectively. The CRDS system consists of two planoconcave high reflectivity mirrors ([greater than] 99.99 percent) at these wavelengths. These are separated by 800 mm and with them we could achieve a maximum ringdown decay time constant of about 160 [mu]s. For time-independent absorbing samples, the enhanced rate of power loss compared to the empty cavity leads to faster exponential decays. When the concentration of the absorbing species changes on the time scale of the empty cavity ring-down time, non-exponential decays result, for which the instantaneous decay rate in excess of the empty cavity reference case provides a time-resolved measure of the sample absorbance. S. Brown et al. recently introduced a method, simultaneous kinetics and ringdown (SKaR), where a single background normalized ringdown is applied to follow the rate of the reaction. We successfully combined the unform flow with the SKaR technique for the first time. We choose vibrationally excited CN formed by photolysis of cyanogen bromide (BrCN) using an excimer laser operated at 248 nm (70 mJ/pulse) as our primary radical species of interest. This molecule has relatively strong transitions in the frequency range of the DFB laser and is a highly reactive radical so it makes an excellent candidate to demonstrate the capabilities of the instrument. We have performed detailed examination of the rate of reaction of vibrationally excited CN(v=1) with O2, NO and butadiene isomers at temperatures 70 K and 24 K. The reaction of CN(v=1) + O2 proceeds via association of the reactants to form a [NCOO] complex which then mainly follows through a lowest energy pathway leading to elimination of an O atom instead of re-dissociation or reaction to NO + CO. On the other hand, for the reaction of CN(v=1) + NO mainly follows through a pathway which leads to dissociation of the complex [NCNO], where it leads to vibrational relaxation of CN through a barrierless pathway. We have measured the rates for these reactions at both 24 and 70 K temperatures and they are in line with the experimental and theoretical calculations found in literature. Another reaction of interest is the reaction of CN(v=1) with butadiene isomers. Both experimental and theoretical evidence suggests the isomers 1,2 and 1,3-butadiene enters through a barrierless PES to form long lived C5H6N complexes. We have measured the rate of reaction for both reactions and we see a substantial difference in their reaction rate. The rate of reaction for the 1,3-butadiene is in excellent agreement with the reported rates for the reaction with (v=0), suggesting no evidence of vibrational enhancement. The related reaction of CN with 1,2-butadiene at low temperatures has not been studied, to our knowledge.