Intracavity infrared-microwave double-resonance spectroscopy

The paper discusses the development of the intracavity infrared-microwave (or radio-frequency) double-resonance method in which a microwave resonance of low-pressure gas placed inside a laser cavity is detected as a variation of laser power. Emphasis is laid on the very high sensitivity of this method and on the three phenomena observed by this method, i.e. (1) the transfer of saturation holes and spikes to other levels that are not directly pumped by the laser through weak intermolecular interactions without a change in velocity, (2) velocity-tuned multiphoton processes, which complicate the structure of saturation holes, and (3) the off-resonant ‘double’ resonance in which microwave pumping is detected through the dispersive response of the laser rather than absorption.