Two highly sensitive microwave cavity spectrometers

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Abstract
A description is presented for two unconventional highly sensitive spectrometers operating in the decimeter‐wave region (90–450 MHz) and millimeter‐wave region (90–150 GHz), respectively. In both cases a resonant cavity is used as an absorption cell. Either design combines high sensitivity with high resolution and wide spectral range. The radio frequency spectrometer allows measuring of extremely low absorption quantity 3×10−12 cm−1 in combination with high spectral resolution of order 1 kHz with the radio frequency–microwave double resonance method. The gas cell is a coaxial cavity (Q≊103) with a diameter of 40 cm and length about a wavelength. The millimeter‐wave spectrometer utilizes an orotron oscillator as the tunable, coherent source of radiation from 90 to 150 GHz. A gas cell is placed into a high quality (Q≊104) Fabry–Pérot resonator of the orotron, and the absorption signal is detected by variation of the orotron electron current in the collector circuit. The sensitivity limit of (2–5)×10−10 cm−1 is achieved with Stark modulation as well as source frequency modulation. Some results of successful application of both designs in molecular spectroscopy are presented, and potential use of the millimeter‐wave cavity spectrometer as a gas analyzer with a detection limit better than 1 ppm is briefly discussed.