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SEPARATION OF A GAS MIXTURE BY A SEMIPERMEABLE OPTICAL PISTON

H. G. C. Werij, J. P. Woerdman, J.J.M. Beenakker, Ivan Kuščer

Published Jun 18, 1984• 0 citations
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Abstract

We have observed effective separation of Na vapor from a buffer gas using the principle of laser induced drift1. The basic idea is as follows. A travelling laser beam produces velocity-selective saturation of optically absorbing atoms, by means of the Doppler effect. This yields opposing fluxes of excited and unexcited atoms. Both fluxes feel different diffusion resistances in the buffer gas, since the collision cross sections σ* and σ of the absorbing atoms are generally state-dependent. This results in a net flux of the absorbing atoms. It should be noted that this mechanical action of the light field is completely different from (and much stronger than) the well known radiation pressure.2

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NASA STI/Recon Technical Report N•Sep 1, 1983•0 citations
We have observed effective separation of Na vapor from a buffer gas using the principle of laser induced drift1. The basic idea is as follows. A travelling laser beam produces velocity-selective saturation of optically absorbing atoms, by means of the Doppler effect. This yields opposing fluxes of excited and unexcited atoms. Both fluxes feel different diffusion resistances in the buffer gas, since the collision cross sections σ* and σ of the absorbing atoms are generally state-dependent. This results in a net flux of the absorbing atoms. It should be noted that this mechanical action of the light field is completely different from (and much stronger than) the well known radiation pressure.2
Na atoms in a buffer gas are compressed to a steep front by a laser beam tuned into the red Doppler wing of the Na DDD transition. The Na densities on either side of the front differ by a factor of 500. The front formation is due to a laser-induced drift which is a consequence of the state dependence of the diffusion cross sections involved. The Na drift velocity is deduced to be \ensuremath{\sim} 4 m/s.
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.
Light-induced drift of optically active atoms in a buffer gas is a consequence of the unequal diffusive frictions suffered by the excited and the ground-state atoms. This drift can be used to create an ``optical piston,'' in which the active atoms are pushed forward by light through the semipermeable membrane of the buffer gas. Normally, optical piston effect is studied when the active atoms are neutral in a confined one-dimensional situation. We present a detailed theory of this phenomenon when the active atoms are charged and a magnetic field is applied for ``tuning'' the direction as well as the magnitude of the drift, thus removing the necessity of confinement. Our study is in different geometries of the light beam and the magnetic field, first in the weak collision model, and then for the strong collision and the Boltzmann-Lorentz models.
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.
The use of highly monochromatic light permits the selective excitation of atoms in vapors if excitation and detection of the fluorescence is carried out collinearly. The atoms capable of absorbing light then form an atomic beam of well-defined velocity along the direction of the laser beam, but no velocity selection occurs perpendicular to it. The potential of the technique for Doppler-free atomic spectroscopy and for the study of excited atom collisions is demonstrated by using the Na D1 line as an example.
The use of highly monochromatic light permits the selective excitation of atoms in vapors if excitation and detection of the fluorescence is carried out collinearly. The atoms capable of absorbing light then form an atomic beam of well-defined velocity along the direction of the laser beam, but no velocity selection occurs perpendicular to it. The potential of the technique for Doppler-free atomic spectroscopy and for the study of excited atom collisions is demonstrated by using the Na D1 line as an example.