Exploration of radiative properties of very excited atoms

Abstract
Various experiments involving very excited atoms and millimetre-wave photons are described, including high-resolution spectroscopy of Rydberg transitions, Rabi nutation and transient maser action between Rydberg levels. The large number of extremely narrow Rydberg spectral lines observed throughout the millimetre-wave spectrum provide a very good ‘calibration' of the scale of Rydberg levels of Na and Cs near the ionization limit, which could be used for the realization of precise infrared frequency markers. The observation of Rabi nutation and maser signals involving an unusually small absolute number of atoms and photons reveals the very great strength of the Rydberg atom to radiation coupling and is a clear indication that these atoms could be used as extremely sensitive millimetre-wave detectors. At a more fundamental level, the study of transient Rydberg masers provides a test of superradiance theory. Extrapolation of these maser experiments to the investigation of a situation where a single atom would exchange a single millimetre-wave photon with a resonant cavity is considered.