The dependence of the Rayleigh-type optical mixing signal intensity on the incident pump and probe frequencies for different values of T1/T2 is shown in a three-dimensional plot for a homogeneously broadened line. The symmetry properties of the signal in this space and of the nonlinear process that generates the line are related. These properties are used to show the dependence of the measured spectra on the way in which the frequencies of the incident fields are varied. Furthermore, this plot in frequency space yields information that may be used for the determination of the relaxation times T1 and T2. © 1990, Optical Society of America.