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Physical Review A

Complete Doppler coverage in laser optical pumping by wall-induced velocity-changing collisions

Charles H. Holbrow, A. P. Ghosh, Daniel J. Heinzen, X. Zhu, W. W. Quivers, Gregory M. Shimkaveg, P. G. Pappas, John E. Thomas, Michael S. Feld

Published Sep 1, 1986• 15 citations
Publisher

Abstract

Enhanced Doppler coverage has been observed in the laser-induced optical pumping of low-density rubidium vapor in the absence of any buffer gas. Transient and steady-state measurements show that this enhancement arises from velocity-changing collisions (VCC's) of rubidium atoms with the walls of the cell containing the vapor. Wall-induced VCC's offer a possible mechanism for increasing the efficiency with which atomic and nuclear polarization may be produced with lasers.

DOI
10.1103/physreva.34.2477

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Physical Review A•Jun 14, 2011•14 citations
Enhanced Doppler coverage has been observed in the laser-induced optical pumping of low-density rubidium vapor in the absence of any buffer gas. Transient and steady-state measurements show that this enhancement arises from velocity-changing collisions (VCC's) of rubidium atoms with the walls of the cell containing the vapor. Wall-induced VCC's offer a possible mechanism for increasing the efficiency with which atomic and nuclear polarization may be produced with lasers.
Experiments have shown the novel combination of laser optical pumping and velocity-changing collisions (between active- and buffer-gas atoms) to be very effective in producing vapors with high atomic or nuclear polarizations. This paper presents a theory which has proven successful in analyzing these experiments. The theory predicts the amount of polarization attainable for particular experimental conditions and establishes the general criteria for optimum polarization. The theory is based on sets of coupled rate equations, which contain terms describing the pumping process (i.e., stimulated absorption or emission), radiative branching, relaxation, and velocity-changing collisions. The collision terms are written in the strong-collision approximation, which assumes that a single collision, on average, thermalizes the velocity distribution. Expressions are derived for the polarization and level populations.
We extend the theory of velocity-selective laser optical pumping in atomic vapors, allowing for arbitrarily rapid velocity-changing collisional rates. The results yield a simple relationship between the line-shape areas of narrow spectroscopic features present in optically pumped samples. This relationship gives directly the ground- and excited-state velocity-changing collision rates and cross sections. We use the results to analyze the data from a laser optical-pumping experiment in 6Li^{6}\mathrm{Li}6Li vapor perturbed by Ar buffer gas.
The Diode Pumped Alkali Laser (DPAL) is an optically-pumped three-level laser system with significant promise for scaling to high power. The rates for velocity changing collisions in the rubidium laser have been measured using sub-Doppler laser saturation spectroscopy. Spectra of the F”=2 to F’=1,2,3 hyperfine components of the Rb S1/2 – P3/2 D2 transition were observed for argon pressures of 0-110 mTorr and modulation frequencies of 700 3200 Hz. Sub-Doppler features with linewidths of about 23 MHz, or about four times the natural linewidth, were observed for the hyperfine transitions and cross-over resonances. A Doppler pedestal is also observed which increases with pressures and decreases with modulation frequency. The size of the Doppler pedestal relative to the homogeneous features yields a rate for reversing the direction of all velocity groups of 3.2 x 10 cm s.
We report new results on the effect of collisions on radio-frequency (rf) -laser double resonances in an atomic vapor under conditions of velocity-selective optical pumping. Our experiments are performed on Zeeman resonances in the 4${f}^{6}666{s}^{2}$ 7{\mathrm{}}^{7}7F1{F}_{1}F1​--4${f}^{6}$6s6p 7{\mathrm{}}^{7}7F0{F}_{0}F0​ transition in samarium vapor in the presence of rare-gas perturbers (He, Ne, Ar, Xe). The collisional relaxation of the rf-generated sublevel coherence is studied using Raman heterodyne detection; this recently introduced spectroscopic technique is capable of separately yielding signal contributions originating from optically resonant and off-resonant atoms. The corresponding measurements clearly reveal that velocity-changing collisions (VCC) can completely determine the characteristics of the rf-laser double-resonance signals. Our experimental findings on the linewidths and line shapes of the rf resonances are in satisfactory quantitative agreement with theoretical predictions that are based on a novel four-level density-matrix calculation; here a Keilson-Storer collision kernel is used to describe the velocity diffusion processes. The comparison of calculated and measured results also permits us to derive cross sections for VCC and depolarizing collisions; moreover, information on the spectrum of the velocity changes connected with alignment-preserving collisions can be obtained.
Enhancement and reduction of the fluorescence intensity in Rb atomic vapor are observed. The reduction is due to a depopulation of the 5P - {3/2 } state upon illumination of a Rb vapor cell with light corresponding to the 5P - {3/2 }→5D - {5/2 } transition. Collisional transfer of the 5D - {5/2 } atoms to the 5D - {3/2 } state, followed by radiative decay to the 5P - {1/2 } state, creates an increase in the population of the 5P - {1/2 } state. The increase in the excited state population results in an increase in the fluorescence intensity of the 5P - {1/2 }→5S - {1/2 } transition. The laser-induced population changes are monitored by the decrease or increase of the corresponding fluorescence signals. Using the steady-state rate equations, the rate constants for the stimulated emission(i.e., 5D - {5/2 }→5P - {3/2 }) is 2.0×10 +8s + {-1 }. The cross section for the fine-structure changing collision(i.e., 5D - {5/2 }→5D - {3/2 }) is 6.4×10 + {-14 }cm +2.