Chirped-pulse fourier transform microwave spectroscopy in pulsed uniform supersonic flows

This dissertation is focused on the development of a new experimental apparatus that combines two powerful techniques: Chirped-Pulse Fourier-Transform Microwave (CP-FTMW) spectroscopy and uniform supersonic flows. This combination promises a nearly universal detection method that can deliver quantitative isomer, conformer, and vibrational level specific detection; characterize unstable reaction products and intermediates; and perform unique spectroscopic, kinetics and dynamics measurements. Thus, a new high-power Ka band (26 – 40 GHz) chirped pulse spectrometer with sub-MHz resolution was designed and constructed. In order to study smaller molecules, E-band (60 – 90 GHz) capabilities were also added to the spectrometer. A novel strategy for generating a pulsed uniform supersonic flow through a Laval nozzle is introduced. A new high-throughput pulsed piezoelectric stack valve was constructed and used to produce a cold (20 K) uniform flow with large volumes and densities (~1016 molecules cm-3). The uniform flow conditions for two of noble gases (argon and helium) were characterized using impact pressure measurements and rotational diagrams. It was demonstrated that a flow uniformity extending as far as 20 cm from the Laval nozzle exit can be achieved with a single compound turbo-molecular pump to maintain the operating pressure. Two benchmark reactive systems were used to illustrate and characterize the performance of the new apparatus CPUF: the photodissociation of SO2 at 193 nm, for which the vibrational populations of the SO product are monitored, and the reaction between CN and C2H2, for which the HCCCN product is detected in its vibrational ground-state. The results show that the combination also provides insight into the vibrational and rotational relaxation kinetics of the nascent reaction products. CPUF has been used to determine product branching in a multichannel reaction. This work, the CN + CH3CCH reaction was found to yield HCN via a direct H-abstraction reaction, while indirect addition/elimination pathways to HC3N, CH3C3N, and H2C3HCN were also probed. From these observations, quantitative branching ratios were established for all products as 12(5)%, 66(4)%, 22(6)% and 0(8)% into HCN, HC3N, CH3C3N, and H2C3HCN, respectively. The values are consistent with statistical calculations based on new ab initio results at the CBS-QB3 level of theory. New designer chirp schemes were developed for CPUF, targeting broader applications through reduced data acquisition time and enhanced signal.

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