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Physical Review A

White-light-induced drift in a gas with one optically active species

Alan D. Streater

Published Jan 1, 1990• 8 citations
Publisher

Abstract

A new light-induced drift effect is discussed in which a white-light source separates a gas mixture containing a single optically active (absorbing) species and a buffer gas which does not interact with the light. A three-level model with one upper level and two closely spaced lower levels is used for the active atoms, and strong collisions with the more abundant surrounding buffer gas atoms are assumed. The intensity spectrum in the vapor (including the effects of hyperfine pumping), and the resulting drift velocity are calculated as a function of optical depth into the vapor.

DOI
10.1103/physreva.41.554

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A new light-induced drift effect is discussed in which a white-light source separates a gas mixture containing a single optically active (absorbing) species and a buffer gas which does not interact with the light. A three-level model with one upper level and two closely spaced lower levels is used for the active atoms, and strong collisions with the more abundant surrounding buffer gas atoms are assumed. The intensity spectrum in the vapor (including the effects of hyperfine pumping), and the resulting drift velocity are calculated as a function of optical depth into the vapor.
A source of white light (i.e., with a flat spectrum) can separate the components of a gas mixture. This effect, which may have applications in stellar atmospheres, is due to light-induced drift which appears when absorption lines form in the intensity spectrum of the propagating light. We theoretically discuss the effect in its simplest form: white light shining into a gas containing two optically absorbing species and a buffer gas. Transport equations are presented, assuming two-level atoms and ignoring stimulated emission. Several limiting cases are discussed analytically. A numerical solution is also presented and used to estimate the ranges of validity of the analytical solutions.
It is shown that non-laser radiation with a broad-band spectrum can induce a drift of optically absorbing multi-level atoms through buffer gas and separate them. It is not necessary to have a mixture of absorbing species to form an asymmetry in the initially flat light spectrum with respect to the atomic resonances. The spectrum asymmetry required for a velocity-selective excitation of the atoms may be formed in the propagating light due to the difference in absorption indexes at nearby transitions of the same species. A set of kinetic equations for coupled light and particles is presented. Numerical analysis of the problem and estimations are done for several typical cases.
A simple method which allows one to study the influence of all relaxation processes and the pumping rate on the velocity distribution of the two-level atom populations is reviewed. In particular, the author presents a treatment that consistently includes the effects of phase and velocity relaxation on the atomic coherences. This method is applied to describe the drift of absorbing atoms-the effect which is also named light-induced gas diffusion. The results of numerical calculations for the drift velocity are presented and the role of the atomic coherences in the description of this phenomenon is pointed out. The dependence of the drift velocity on various parameters is discussed and the best experimental conditions for observing this effect are suggested.
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.
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.
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.