The theoretical formulation of the nonlinear optical activity is utilized to describe the signal intensity in forward scattering. For cases in which the ratio of the Rabi frequency of an isolated transition to its relaxation rate is less than one, a perturbation procedure in this parameter is presented. The successive terms in the resulting series are interpreted as higher-order coherences or induced multipole moments. Application is made to the hyperfine components of the sodium D transitions where experimental data is described by a third-order calculation. Good agreement is obtained without invoking an induced hexadecapole moment as was proposed originally.

• Received 12 March 1985

DOI:https://doi.org/10.1103/PhysRevA.32.2223