We describe development of a new instrument combining two powerful techniques: chirped-pulse Fourier-transform microwave spectroscopy and pulsed uniform supersonic flows.  The objective is to join the virtues of microwave spectroscopy: quantitative, near universal detection affording structural information, with Laval flows offering thermalized conditions at low temperature and high density, to study reaction dynamics of polyatomic molecules with unprecedented detail in product characterization. We have constructed the new high-power Ka-band, 26�40 GHz, chirped pulse spectrometer with sub-MHz resolution, analogous to the revolutionary Chirped-Pulse Fourier-Transform MicroWave (CP-FTMW) spectroscopic technique developed in the Pate group at University of Virginia. In order to study smaller molecules, the E-band, 60 � 90 GHz, chirped-pulse capability was also added to our spectrometer. A novel strategy for generating uniform supersonic flow through a Laval nozzle is also introduced. A high throughput pulsed stacked piezoelectric valve is used to produce cold (30 K) uniform flow with large volumes of ~150 cm3 and ~1014 molecules cm-3 with modest pumping requirements. Two competing design considerations are critical to the performance of the system: a low temperature flow is needed to maximize the population difference between rotational levels, and high gas number densities are needed to ensure rapid cooling to achieve the uniform flow conditions. At the same time, collision times shorter than the chirp duration will give inaccurate intensities and reduced signal levels due to collisional dephasing of the free induction decay. Details of the instrument and recent results will be presented.

Chirped-pulse microwave spectroscopy in pulsed uniform supersonic flows: Probing reaction dynamics with rotational spectroscopy

248th National Meeting (2014)

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