  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.
