Supersonic-jet cryogenic-resonator coaxially oriented beam-resonator arrangement Fourier transform microwave spectrometer

The design and performance of a cryogenic (T=77 K) microwave spectrometer are reported. This instrument, being a time domain experiment in microwave molecular spectroscopy, consists of three basic components: a near-confocal Fabry-Perot-type resonator, a pulsed microwave excitation Fourier transform system, and a pulsed molecular-beam source in a coaxially oriented beam-resonator arrangement. With the recently developed spectrometer, which employs confocal reflectors cooled to liquid-nitrogen temperature, we were able to reduce the system noise of the instrument significantly. The high sensitivity of this next generation spectrometer allows the detection of transient species produced in a dc-discharge nozzle, other short-lived molecules and complexes at low abundance, or nearly nonpolar molecules. We will discuss the spectrometer design with respect to critical considerations of the resonator theory in the context of cryogenic operation, as well as important aspects of classical circuit theory applied to excitation and detection of molecular systems in a resonator. Following the description of the dc-discharge source, the performance of the instrument is demonstrated. (c) 2005 American Institute of Physics.