Collisionally induced four-wave mixing in the nonimpact regime

Experiments were performed on the Na-He and Na-Xe systems with degenerate four-wave mixing (FWM) signals observed for detunings as large as 200 cm^-1 from the D-line resonances. Analytical expressions were derived for the FWM intensity as a function of laser intensity, detuning, Na density, and buffer gas pressure. The expressions are correct to all orders of the incident laser intensity, and the observations show that terms up to χ⁽⁷⁾ in the susceptibility are important in the experiments. The expressions depend on the absolute collisional line shape of the Na-perturber system and thus enable the line shape to be determined from the FWM intensity measurements. A unique absolute collisional line shape was determined for Na-He that fits all the experimental data. Results for the Na-Xe system are also shown. For laser detunings to the blue side of D₂ the behavior is similar to that of Na-He, except that the absolute FWM intensity is an order of magnitude lower. A single and unique set of absolute line shape parameters fits all the data. However, satellites are expected for laser detunings to the red side of D₁.