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Quantum Electronics and Laser Science Conference

Collisional potential determination near an anticrossing from four-wave mixing

Boris Vaynberg, M. Matusovsky, M. Rosenbluh

Published Jun 7, 1996• 0 citations
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Abstract

Summary form only given. The collisional potentials of Na and Xe are determined from a four-wave mixing (FWM) experiment. These potentials are shown schematically for large Na-Xe distances, the transition energies of the Na atom approach the unperturbed resonances corresponding to the Na D lines. The Na-Xe diatomic molecular potentials have an avoided level crossing, which can lead to satellite structure in the absorption line shape (ALS). In collisional FWM the intensity of the generated FWM signal is determined by the atomic transition ALS.

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Physical Review A•Nov 1, 1984•36 citations
Summary form only given. The collisional potentials of Na and Xe are determined from a four-wave mixing (FWM) experiment. These potentials are shown schematically for large Na-Xe distances, the transition energies of the Na atom approach the unperturbed resonances corresponding to the Na D lines. The Na-Xe diatomic molecular potentials have an avoided level crossing, which can lead to satellite structure in the absorption line shape (ALS). In collisional FWM the intensity of the generated FWM signal is determined by the atomic transition ALS.
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noble gas is measured and calculated for large detunings from resonance, where the collisional impact approximation is no longer applicable.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.
Collisionally induced four-wave mixing (FWM) signals have been observed in Na vapor broadened by He buffer gas at large detunings from resonance. Analytical expressions are developed which do not use the collisional impact approximation and which accurately describe the dependence of the FWM intensity on laser detuning and incident laser intensity. The measurements and the theory are used to calculate the nonimpact collisional lineshape for the Na-He system for detunings of 40 to 200 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ from the D line resonances.
Study of a two-photon near resonant four wave difference mixing (FWDM) in Na in a CH4/air flame has been performed to understand the FWDM process reported by Singh and Yueh (1992). The FWDM signal was measured with reduced flame length and with the focal point at the center and also at one end of the flame to pinpoint the region in which the signal is generated. A partial contribution in the third peak was from the nonresonant background. A FWDM signal is generated slightly away from the focal point and nonresonant background, at the focal point in the present experimental conditions. An absorption spectrum of the Na D-lines at different dye laser energy was recorded to study the depletion of Na ground state at higher dye laser intensity. Comparative study of the FWDM signal with a narrow and broad bandwidth dye laser shows that a lower Na concentration detection limit could be achieved with a narrow linewidth probe laser tuned to about 9/cm lower frequency from the center of the D1 line. The two minima in the FWDM spectrum corresponding to Na D lines have been explained, qualitatively, with the saturation effect in the Na D-lines transition. 27 refs.
Degenerate four-wave mixing (DFWM) in excited atoms is investigated as a probe of the excited-state density produced by collision-assisted transitions. Sodium atoms are excited to the 3P state by absorption of light in the extreme blue wing of the D lines in the presence of He, Ne and Ar perturbers. The rare-gas pressure dependence of the DFWM signals, resonantly enhanced by the 3P-4D transition, is studied to determine the regime where the signals give an unambiguous measure of the excited-state (3P) density. The observed behaviour is compared with a simple model of DFWM in absorbing media. Good qualitative agreement is obtained and the results illustrate the role of absorption, saturation and finite laser bandwidth. The technique allows small excited-state densities ( approximately 10 9 cm -3 ) to be detected with good temporal and spatial resolution.
Optical four-wave mixing resonances in sodium vapor induced by collisions with inert buffer gases (He, Ar, Xe, N2{\mathrm{N}}_{2}N2​) are characterized at high resolution ( 5 MHz) using lasers tuned near the DDD lines of Na. These resonances are interpreted in terms of coherent scattering from a population grating established by collisional absorption of the incident radiation. The roles of homogeneous and inhomogeneous contributions to the resonance line shapes are clarified. The line shapes exhibit structure sharper than the predicted linewidth, that characteristic of the 3P3P3P-to-3S3S3S transition rate. We show that this is caused by collision-induced optical pumping which introduces the ground-state lifetime into the grating relaxation physics. These long-lived gratings are exemplified dramatically by experiments in the presence of N2{\mathrm{N}}_{2}N2​ which collisionally quenches the Na 3P3P3P states. Similar results are obtained in Xe where it is argued that excimer formation with the 3P3P3P states plays a role in the population dynamics. Model calculations of the effects of sodium level degeneracy on the grating dynamics are presented to support these conclusions. Finally we present data illustrating interference between the collision-induced population grating resonances detailed here and collision-induced coherent Raman resonances between Zeeman sublevels of the ground state.