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Physical Review A

Numerical study of light-induced drift of Na in noble gases

Jos E. M. Haverkort, H. G. C. Werij, J. P. Woerdman

Published Oct 1, 1988• 25 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.38.4054

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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).

Conference on Lasers and Electro-Optics•Apr 25, 1988•0 citations
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.
We report on an experimental investigation of the collision kernels governing the effects of velocity-changing collisions, fine-structure-changing collisions, and their interplay, for Na--noble-gas pairs. The velocity distributions in the two Na excited-state fine-structure levels and in the Na ground-state hyperfine levels have been measured in the presence of a pump laser which produces velocity-selective excitation. For interpretation of these experiments we use a rate-equation model developed in the context of light-induced drift: the velocity-changing collisions are accounted for by a composite Keilson-Storer kernel describing large-angle and small-angle collisions and the fine-structure-changing collisions are treated in the sudden limit. The model is valid for arbitrarily high pump laser intensities and uses only a single adjustable parameter characterizing the small-angle scattering. It is found that the theory describes the experimental velocity distributions in all four Na levels over a wide range of experimental parameters for each Na--noble-gas pair. The relevance of the collision kernels deduced in the present work is discussed in the context of light-induced drift.
We report an experimental and theoretical study of the optical piston, a consequence of the phenomenon of light-induced drift (LID), observed in binary mixtures of Na vapor and a buffer gas. Good agreement has been found between experiment and existing theory as far as the shape of the Na density profile in the piston is concerned. The piston velocity is anomalously low; this is accounted for by incorporating the effects of adsorption and desorption into the theory. Furthermore, we introduce a rate-equation model for LID which incorporates the fine and hyperfine structure of the level scheme of the Na absorbers; this gives insight into the sublevel velocity distributions and the nonequilibrium part of the total velocity distribution during LID.
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).