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Journal of Physics B Atomic Molecular and Optical Physics

Light-induced drift under conditions of ground state optical hyperfine pumping

Sergey Nikitich Atutov, А. И. Пархоменко, S.P. Pod'yachev, Anatolii M. Shalagin

Published Jul 14, 1992• 5 citations
Publisher

Abstract

Analytical expressions describing the light-induced drift (LID) of alkali metal vapours reasonably well both under conditions of ground state optical hyperfine pumping and in its absence are obtained. It is shown that under the conditions of optical pumping the LID effect of the particles arises under the action of its own fluorescent radiation. The drift velocity for sodium atoms in a argon atmosphere reaches a value of 1 cm s -1 for fluorescent radiation of intensity 20 mW cm -2 .

DOI
10.1088/0953-4075/25/13/004

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Physical Review A•Aug 1, 1990•6 citations
Analytical expressions describing the light-induced drift (LID) of alkali metal vapours reasonably well both under conditions of ground state optical hyperfine pumping and in its absence are obtained. It is shown that under the conditions of optical pumping the LID effect of the particles arises under the action of its own fluorescent radiation. The drift velocity for sodium atoms in a argon atmosphere reaches a value of 1 cm s -1 for fluorescent radiation of intensity 20 mW cm -2 .
Light-induced drift (LID) is a transport phenomenon in a gas mixture with dramatic manifestations such as the optical piston1 and applications in atomic isotope separation.2 The induced drift velocity is proportional to the excited state fraction, and for Na in Xe high drift velocities have been achieved using two lasers.3 Here one laser excites atoms from one hyperfine ground state and the second laser atoms from the other, avoiding hyperfine pumping. Excited in this way, the Na atom represents a Λ type three-level system, where coherent effects can be important.
The anomalous light-induced drift (LID) of atoms caused by a velocity dependence of collision frequencies has been investigated in the frame of the theory without adjustable parameters. Our results open the possibility of experimental probing of models of interatomic potentials. The extension of the strong-collision model to the case of velocity-dependent collision rates, is the basis of our approach. The obtained model describes the anomalous LID both in the case of arbitrary mass ratio of absorbing- and buffer-gas particles and in the case of arbitrary ratio of the homogeneous and Doppler widths. In particular, we applied our model to describe the anomalous LID in alkali - noble-gas mixtures (Rb - Kr and Li - Ne systems). Qualitatively another anomaly of the LID velocity was also found in the low-pressure region. As pointed out, the optical pumping effect and the hyperfine structure of the ground electronic state, but not the velocity dependence of the collision rates, are the sources of this new anomaly.
The use of paraffin-coated cells has led to a breakthrough in the experimental study of light-induced drift (LID) of alkalis in noble gases. In recent experiments isotope separation of the two naturally occurring Rb isotopes through the LID effect has been achieved (enrichment of the rare isotope up to 95%), 1,2 and direct measurements of the drift velocity of Na in noble gases as a function of various experimental parameters have been made.3 In these experiments a single single-mode dye laser, tuned within the Doppler profile of the resonance line, was used to excite a specific velocity group of atoms (isotope selective in the case of Rb). Optical pumping between the ground-state hyperfine levels F = I+½ and F=I−½ plays a dominant (limiting) role in determining the magnitude of the LID effects by strongly depleting the resonantly excited hyperfine substate (of a specific isotope).
We present a model for light-induced drift (LID) in the Na--noble-gas system which should enable direct comparison with experiment. In contrast to previous theories of LID based on a two-level description of the optical absorbers and on a simplified collision treatment, the present model is based on a realistic description of laser-driven Na atoms immersed in a buffer gas. Starting from the generalized Bloch equations, we introduce a rate-equation model for the velocity distributions in the four important Na levels. The velocity-changing and fine-structure-changing collisions are described using composite Keilson-Storer collision kernels in which all adjustable parameters have been eliminated by using available literature data. We apply the model in numerical calculations of LID as a function of all experimentally accessible parameters. It is found that the ground-state hyperfine splitting can have large effects on LID, whereas the excited-state fine-structure splitting has not. The paper establishes criteria for optimum LID effects; when using a single-frequency laser the maximum attainable drift velocity is predicted to be 13.8 m/s. Using a proper set of boundary conditions, we find a pressure dependence of LID qualitatively different from the predictions based on previous work. Finally, the influence of the collision model is investigated. We find that LID is independent of the shape of the collision kernel, indicating that a strong-collision model is always valid.
The influence of the bandwidth of the radiation source on light-induced drift (LID) of Na is studied experimentally. Broadband radiation can be used to eliminate optical hyperfine pumping on the one hand, and to provide more efficient excitation due to an increased velocity coverage, on the other hand. These aspects are highlighted in two separate experiments. An increase of the drift velocity of Na by a factor of 4 compared to monochromatic excitation has been measured. A frequency-modulated (FM) ring dye laser is used as a broadband radiation source, having a bandwidth continuously variable from single mode to multimode with a bandwidth of 10 GHz. Contrary to passive multimode lasers, the spectrum of such a laser is well defined and stable. Various modulation frequencies are used to study the dependence of the drift velocity on the mode spacing of the multimode laser. Only small differences are found. All experimental results are compared with results of a four-level rate-equation model for LID of Na, in which the excited-state hyperfine structure of Na and the detailed shape of the FM spectrum are taken into account. Good agreement between the model and the experimental data is found. The model is also used to show that the FM spectrum yields almost the same values for the drift velocity as a rectangular spectrum, which so far has been considered optimal for LID.