Satellite structure in the Na-Xe collisional lineshape observed in four wave mixing

Degenerate four-wave mixing in sodium vapor in the presence of a noble gas is measured and calculated for large detunings from resonance, where the collisional impact approximation is no longer applicable. The measurement thus probes the collisional potential of the Na-noble gas system. When the noble gas is Xe, and the detuning is to the red of the Na D resonance, an avoided crossing between the quasi-molecular potentials results in a satellite in the absorption line shape, which can be detected in the four wave mixing signal. We calculate collisional line shapes with several available sets of Na-Xe interaction potentials using the quasi-static and semi-classic line broadening theories, and find that the available potentials do not yield line shapes which agree with the measurements. The lineshape derived from the measured degenerate four wave mixing signal shows good agreement with theoretical calculations, only if the non-adiabatic nature of the leve crossing is considered. Based on our experimental lineshape and a non-adiabatic level crossing theory, we derive corrections to the Na-Xe potentials, near the level crossing, that is for interatomic separations in the range of 0.55-0.75 nm.