Nonlinear interference effects in emission, absorption, and generation   spectra
  Nonlinear effects in emission and absorption spectra of gaseous systems are considered. It is shown that level splitting can be detected spectroscopically even if it is below the Doppler width. Conditions for distinguishing interference effects from those due to nonequilibrium velocity distribution are determined. In the case of large Doppler broadening the correction for atomic motion is equivalent to the substitution of an "effective immobile atom" for the moving atom ensemble. The spectral manifestation of nonlinear effects is analyzed in detail. The influence of nonlinear interference effects on the generation characteristics in the presence of external field is investigated. 
T. Ya. Popova, A. K. Popov, S. G. Rautian, and R. I. Sokolovskii