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Chinese Physics Letters

Observation of collisional narrowing by time-resolved degenerate four-wave mixing in sodium

Zuhe Yu, Peixian Ye, Panming Fu

Published Jun 1, 1986• 0 citations
Publisher

Abstract

The first observation of collisional narrowing by time-resolved degenerate four-wave mixing has been reported. The velocity-changing collision cross section of Na in Ar we obtained is 5.6×10 -15 cm 2 .
DOI
10.1088/0256-307x/3/6/001

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Recently, we have shown that the nearly degenerate four-wave mixing (NDFWM) process can provide a direct and simultaneous measurement of the longitudinal (1/T1) and transverse (1/T2) relaxation rates of a resonant two-level atom. Furthermore, we observed that in the presence of buffer gases, the linewidth of the resonance describing the longitudinal relaxation process experiences a significant narrowing, belowo that of the natural width. Using a theoretical model, the narrowing was accounted for as arising from collisional effects in the ground-state population. 1 These observations were made for the case where the radiation fields are co-polarized, and we concluded that NDFWM offers a unique technique for the measurement of the ground-state velocity-changing cross section.2
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The spectral behavior of the third order nonlinear optical response of atomic sodium was studied. As a result, new kinds of frequency domain spectroscopy techniques were developed: nearly degenerate four-wave mixing, polarization nearly degenerate four-wave mixing, frequency domain three-state spectroscopy, and nonlinear Hanle effect in degenerate four-wave mixing. These techniques have provided new information on several areas of collision physics including: dephasing collisions (spectral broadening and frequency shift), velocity changing collisions, state changing collisions, and spin-flip collisions. (Author)
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noble gas is measured and calculated for large detunings from resonance, where the collisional impact approximation is no longer applicable.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 non-impact 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 1 resonance, an avoided crossing between the quasi-molecular potentials results in a satellite in the absorption lineshape, which can be detected in the four wave mixing signal. We calculate collisional lineshapes 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 level 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.