About nondegenerative four photon mixing gain spectroscopy of the SF_6   molecules
  Infrared-infrared double resonance measurements of the $\nu_3$ absorption band of $SF_6$ using a $CO_2$ TEA laser radiation have been carried out. The results obtained in the present and in some earlier works are discussed taking into account existence of a wide component of lines, nondegenerative four photon mixing (NFPM) and time invariance violation in the photon-molecule interactions. It is proposed to use the NFPM effect in investigations of the shape of the optical transition in polyatomic molecules and especially for evaluation of the homogeneous width of the wide component of lines. The need and problems of experimental study of existing effects of the T-invariance violation in optics are discussed. 
V.A.Kuz'menko