Low temperature reaction kinetics inside an extended Laval nozzle : REMPI characterization and detection by broadband rotational spectroscopy
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The importance of reaction kinetics at low temperatures cannot be overstated, for its impact on human understanding of atmospheric and astrochemical environments is fundamental. Bringing the study of these interactions from nature into a controlled laboratory setting where they can be measured is no simple feat. The coupling of CRESU (a French acronym, cinetique de reaction en ecoulement supersonique uniforme or "Reactions Kinetics in Uniform Supersonic Flow") with the revolutionary Chirped-Pulse Fourier Transform Microwave Spectroscopy (CP-FTMW) method forming what has been dubbed CPUF (Chirped-Pulse/ Uniform Flow) has allowed just this to occur. A cold, uniform column of gas is obtained via the use of a Laval nozzle and measured using CP- FTMW. Instrumental sensitivity is best achieved at low temperature and pressure conditions for this setup, but obtaining and maintaining these conditions is experimentally challenging. Sampling methods such as airfoil and skimmer have been used previously by the Suits group and others. Here, a new solution, a Laval nozzle extension, was developed and implemented. Reactions take place within the nozzle, after which a second expansion occurs. This shock-free secondary expansion provides a low-density region of gas to examine, ideal for CP-FTMW detection. The flow resulting from this expansion from the nozzle extension was characterized via resonance enhanced multi-photon ionization (REMPI). This approach was then applied to the reactions, HCO + NO and HCO + O2, with their rate coefficients being measured for the first time under low temperature conditions. This thesis describes the development and use of that nozzle as well as relevant experimental and theoretical data necessary in its implementation.
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M.S.