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A highly sensitive intracavity millimeter-wave spectrometer was developed for the investigation of the absorption spectra of van der Waals complexes in a supersonic jet. The key element of the spectrometer is a tunable oscillator, called OROTRON, which generates the millimeter-wave radiation through the interaction of an electron beam with the electromagnetic field of a high quality (Q≈104) Fabry–Perot resonant cavity. This cavity consists of a movable spherical mirror and a fixed planar mirror with the periodic structure imprinted on its surface. The electron beam moves along the periodic structure of the plane mirror. This part separated from the rest of the resonator by a mica foil is kept under ultrahigh vacuum conditions. The molecular jet is injected by a pulsed valve into the other part of the resonator. The absorption in the jet is sensitively detected by measuring the electric current in a special collector circuit of the OROTRON. The spectral purity of the OROTRON radiation is 10–15 kHz providin...

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–Perot 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...