Resonant spectral structures of four-wave mixing (FWM) in molecular sodium
are investigated. For a double- configuration with strong-weak-strong-weak
fields, split components in FWM spectra induced by the strong pump fields a
re observed. It is shown that the split components merge into a single peak
for a certain ratio of the strong field intensities. A correlation between
the level-splitting effects and the saturation behavior of the FWM signal
is experimentally demonstrated. (C) 2001 Optical Society of America.