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An atom which possesses one valence electron orbiting about an atomic nucleus within an electron shell well outside all the other electrons in the atom. Such an atom approximates the hydrogen atom in that a single electron is interacting with a positively charged core. Early observations of atomic electrons in such Rydberg quantum states involved studies of the Rydberg series in optical spectra. Electrons jumping between Rydberg states with adjacent principal quantum numbers, n and n - 1, with n near 80 produce microwave radiation. Microwave spectral lines due to such electronic transitions in Rydberg atoms have been observed both in laboratory experiments and in the emissions originating from certain low-density partially ionized portions of the universe called HII regions. See Electron configuration
The advent of the laser has made possible the production of sizable numbers of Rydberg atoms within a bulb containing gas at low pressures, 10-2 torr (1.3 pascals) or less. The rapid energy-resonance absorption of several laser light photons by an atom in its normal or ground state results in a Rydberg atom in a state with a selected principal quantum number. Aggregates of Rydberg atoms have been used as sensitive detectors of infrared radiation, including thermal radiation. They have also been observed to collectively participate in spontaneous photon emission, called superradiance. Such aggregates form the active medium for infrared lasers that operate through the usual laser mechanism of collective stimulated photon emission. All these developments are based upon the great sensitivity of Rydberg atoms to external electromagnetic radiation fields. Atoms with n near 40 can absorb almost instantaneously over a hundred microwave photons and become ionized at easily achievable microwave power levels. Isotope separation techniques have been developed that combine the selectivity of laser excitation of Rydberg states with the ready ionizability of Rydberg atoms. Such applications have been pursued for atoms ranging from deuterium through uranium. See Infrared radiation, Laser