  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.
