Formation mechanisms and structure of the luminescence spectra of a dense resonant medium

The purely thermal visible and infrared radiation emitted by a dense resonant medium (sodium vapor) heated nonuniformly to temperatures of 600–1200 K was investigated experimentally for the first time under conditions where the photon mean free path is comparable with the emission wavelength. The profile of the recorded spectra and the absolute luminescence intensities in the different spectral ranges show good agreement with the results of a numerical simulation using a previously developed theory of resonance radiation transport which assumes a Boltzmann spectral distribution of the resonant level population proportional to exp(−ℏω/T). The self-reversed resonant sodium line exhibited strong asymmetry and it was shown that under certain conditions, the luminescence spectrum of the medium may exhibit an additional broad peak on the far “red” limb of the resonance line. Calculations and measurements demonstrated that the intensity of the thermal emission of sodium vapor at this red peak is several orders of magnitude higher than that obtained from the standard theory of resonance radiation transport. This effect is arbitrarily termed an infrared “ catastrophe.” It is noted that in a solar corona plasma and in gas-discharge lamps, the far red limbs of the resonant lines may make a substantial contribution to the total luminescence intensity and in some cases, considerably exceed the intensity of the photorecombination and bremsstrahlung continuum. Zh. éksp. Teor. Fiz. 114, 135–154 (July 1998)