Optically-induced energy level shifts for intermediate intensities

We report measurements of optically-induced energy level shifts produced by nonresonant light at intermediate intensities in atomic sodium vapor. The intensity dependence of the shifts departs substantially from the linear behavior predicted by 2nd order perturbation theory. The behavior is in good agreement with more exact calculations and yields a value of 0.10 for the 3P_{$\text{3}\text{2}$}?4D oscillator strength.