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Physical Review A

Separation of two atomic species by white-light-induced drift

A. K. Popov, Anatolii M. Shalagin, Alan D. Streater, J. P. Woerdman

Published Jul 1, 1989• 13 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.40.867

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Abstract : The effect of light-induced drift (LID) in gases, predicted by Gel'mukhanov and Shalagin and observed later, is manifested in mechanical drift of the absorbing atoms (molecules) in response to optical excitation. The effect of LID takes place if the atom interaction with light is velocity-dependent (due to the Doppler effect), the spectrum of the exciting radiation is asymmetric with respect to the the absorption line, and the absorbing molecules experience collisions with a buffer gas, with the collision rate dependent upon their internal state. The idea of LID is the following. Suppose for the sake of definiteness that the collision frequency of the absorbing molecules with the buffer is greater in the excited state and that the exciting light is red-shifted with respect to the absorption contour. Then due to the Doppler shift, the radiation interacts mainly with the counter-moving molecules, which undergo transitions into an excited state, in which their friction upon the buffer component is increased. Due to this increase, the counter-flying molecules are slowed down, while the molecules moving in the direction of light are not affected. The net effect is a drift of the absorbing component in the direction of light. The drift alternates its direction with the change of signs of either the detaining or the difference in the collision frequencies.

•May 22, 1992•0 citations

Theory of high-field effects in light-induced drift

Faris Gel’mukhanov, Jos E. M. Haverkort, S. W. M. Borst +1 more

We address theoretically the effects of a large laser intensity on the phenomenon of light-induced drift. The velocity selectivity of the atom-field interaction decreases due to saturation broadening and this leads to a maximum of the drift velocity as a function of the laser intensity. Analytical expressions for the drift velocity are derived in the case of two-level atoms, assuming a strong collision model. Consequences of this high-field effect are discussed for various manifestations of light-induced drift such as propagation of a soliton in an infinitely long cell, described by the Burgers equation, and collisions of such solitons. The analogy with hydrodynamic shock waves, described by the same Burgers equation, is fruitfully explored. We also address high-field effects on the density distribution produced by light-induced drift in a finite cell. In all these manifestations of light-induced drift the cases of high field and low field show qualitatively different behaviors which are of a complementary nature.

Physical Review A•Jul 1, 1987•18 citations
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.
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.
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.
The authors calculate the light-induced drift velocity of multi-level atoms in the strong collision approximation. The solution is presented in a general form which can be easily applied to various systems. Specific calculations of drift velocities are presented for the system Rb:Ar for the cases of single- and multi-frequency laser excitations. A calculation is also presented for the case of a white light excitation spectrum with a spectral dip, which could be caused by absorption of light by another substance in the vapour, for instance in stellar atmospheres.
Abstract : The effect of light-induced drift (LID) in gases, predicted by Gel'mukhanov and Shalagin and observed later, is manifested in mechanical drift of the absorbing atoms (molecules) in response to optical excitation. The effect of LID takes place if the atom interaction with light is velocity-dependent (due to the Doppler effect), the spectrum of the exciting radiation is asymmetric with respect to the the absorption line, and the absorbing molecules experience collisions with a buffer gas, with the collision rate dependent upon their internal state. The idea of LID is the following. Suppose for the sake of definiteness that the collision frequency of the absorbing molecules with the buffer is greater in the excited state and that the exciting light is red-shifted with respect to the absorption contour. Then due to the Doppler shift, the radiation interacts mainly with the counter-moving molecules, which undergo transitions into an excited state, in which their friction upon the buffer component is increased. Due to this increase, the counter-flying molecules are slowed down, while the molecules moving in the direction of light are not affected. The net effect is a drift of the absorbing component in the direction of light. The drift alternates its direction with the change of signs of either the detaining or the difference in the collision frequencies.
We address theoretically the effects of a large laser intensity on the phenomenon of light-induced drift. The velocity selectivity of the atom-field interaction decreases due to saturation broadening and this leads to a maximum of the drift velocity as a function of the laser intensity. Analytical expressions for the drift velocity are derived in the case of two-level atoms, assuming a strong collision model. Consequences of this high-field effect are discussed for various manifestations of light-induced drift such as propagation of a soliton in an infinitely long cell, described by the Burgers equation, and collisions of such solitons. The analogy with hydrodynamic shock waves, described by the same Burgers equation, is fruitfully explored. We also address high-field effects on the density distribution produced by light-induced drift in a finite cell. In all these manifestations of light-induced drift the cases of high field and low field show qualitatively different behaviors which are of a complementary nature.