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Physical Review Letters

Optical Magnetic Double Resonance Control of Atom-Molecule Composition of Metal Vapors

Robert A. Bernheim, J. Xu

Published Oct 25, 1999• 2 citations
Publisher

Abstract

Optical orientation of Na atoms in a metal vapor with circularly polarized ${D}_{1}$ resonance radiation results in a shift in the $2\mathrm{Na}\ensuremath{\leftrightarrow}\mathrm{Na}{}_{2}$ gas phase equilibrium in the direction that destroys dimers. Plane polarized ${D}_{1}$ or ${D}_{2}$ resonance radiation produces the opposite effect: an increase in equilibrium dimer density. The atom-dimer reaction can be controlled to favor either products or reactants by simply changing the polarization of the resonance light. Application of a radio-frequency field at the Na atomic Zeeman resonance frequency while optically pumping the atoms with circularly polarized ${D}_{1}$ radiation also permits a magnetic resonance tuning control of either an increase or decrease in dimer density.

DOI
10.1103/physrevlett.83.3394

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Optical orientation of Na atoms in a metal vapor with circularly polarized D1{D}_{1}D1​ resonance radiation results in a shift in the 2Na\ensuremath↔Na22\mathrm{Na}\ensuremath{\leftrightarrow}\mathrm{Na}{}_{2}2Na\ensuremath↔Na2​ gas phase equilibrium in the direction that destroys dimers. Plane polarized D1{D}_{1}D1​ or D2{D}_{2}D2​ resonance radiation produces the opposite effect: an increase in equilibrium dimer density. The atom-dimer reaction can be controlled to favor either products or reactants by simply changing the polarization of the resonance light. Application of a radio-frequency field at the Na atomic Zeeman resonance frequency while optically pumping the atoms with circularly polarized D1{D}_{1}D1​ radiation also permits a magnetic resonance tuning control of either an increase or decrease in dimer density.
In the optical excitation of atoms with polarized light, producing excited atoms, only some of the Zeeman sublevels of the excited state are actually reached, so that large differences of population can be built up between Zeeman sublevels or between hyperfine structure (hfs) levels. This property can be used to detect radio-frequency resonance in optically excited atomic states. These resonances produce a characteristic change in intensity or in the degree of polarization of the light re-emitted. Zeeman intervals, Stark effects, and hfs intervals can be measured in this manner. (The Stark constant of the 63P1 level of Hg and the electric quadrupole moments of the alkali atoms have been obtained in this way.)The technique of “optical pumping” gives a way to concentrate atoms in some of the Zeeman sublevels of one of the hfs levels of the ground state.Atomic orientation has been obtained with the Na atom, in an atomic beam and in the vapor in equilibrium with the metal. The orientation effects have been studied by detection of radio-frequency resonance signals in the ground state. Orientation can be increased many times by adding a variable pressure of a foreign gas to the pure Na vapor. Because of the coupling between nuclear spin and electron spin, nuclear orientation is produced at the same time as atomic orientation.
Optical orientation of reagents is one approach to achieving selectivity in a chemical reaction and has been used successfully to shift the point of equilibrium in bulk reaction mixtures of alkali-metal atoms and dimers. In addition to the continuous control of the point of equilibrium in the dimerization of sodium atoms, the control of the ortho-para nuclear spin distribution is shown here to also be a function of the degree of atomic spin orientation over a broad range. The direction of the spin conversion is para to ortho. For a mutlicomponent vapor, the optical orientation of 6Li{}^{6}\mathrm{Li}6Li in a mixture of lithium-6 and lithium-7 is shown to produce a continuous and selective control of the isotopic dimer concentrations. 6Li2{}^{6}{\mathrm{Li}}_{2}6Li2​ (para) is selectively destroyed in the presence of the other dimer species. In this way the composition of a multicomponent metal vapor system at chemical equilibrium can be manipulated. The constraint produced by the optical orientation of one of the reagents offers a technique whereby the equilibrium yield of a multicomponent bulk chemical reaction can be laser controlled in a selective and continuous fashion.
A shift in the equilibrium distribution between reactants and products in the vapor phase chemical reaction 6Li+7Li=6Li 7Li is caused by the optical orientation of atomic Li spin angular momenta and is shown to be controlled by the degree of atomic orientation. The observations were carried out in a heat-pipe cell operated at 631 K and containing a He buffer gas density of 3.1×1018 cm−3. The shift is similar to that measured for the ortho molecular components in the 2 6Li=6Li2 and 2 7Li=7Li2 gas phase equilibrium reactions.
Change in polarization produced by circularly polarized D(1) + D(2) radiation and destroyed by the resonance can be observed by monitoring the intensity of the radiation transmitting the vapor. The polarization and intensity of the absorption signal (change in absorption of the radiation produced by the resonance) are calculated from the solutions of equations governing the rate at which the populations of Zeeman sublevels of the ground state change. The result shows that the polarization increases as the disorientation mixing in the excited state decreases, and that the signal intensity is nearly equal to that when the D(1) radiation was used alone. Signal representation derived by Bell and Bloom based on Bloch's equations is modified so as to be applied to the experiment using D(1) + D(2) radiation as well as to that using D(1) radiation alone. The calculated result from the representation shows that the amplitude of the modulation signal (rf light modulation at the resonance) for D(1) + D(2) radiation is also nearly equal to that for D(1) radiation alone.
A variation of the usual method of atomic orientation of alkali metal vapors by optical pumping is proposed. This variation consists of employing a single circularly polarized D1{D}_{1}D1​ line as pumping radiation, instead of a mixture of the two DDD lines. Under these conditions, one magnetic substate of the ground state is transparent to the pumping radiation. It is shown that for a sufficiently intense light source and relatively weak relaxation, the entire population of the vapor will tend to be pumped into this single state, resulting in nearly one-hundred percent atomic orientation. The pumping process is analyzed in detail for sodium vapor illuminated with $3{P}_{\frac{1}{2}}\ensuremath{-}3{S}_{\frac{1}{2}}$ radiation, and the time dependence of the populations of the eight magnetic sublevels of the ground state is computed. It is shown that in the steady state, the light absorption by the vapor is linear rather than exponential with distance. A simple method of measuring relaxation times based on this observation is suggested. The conditions under which the imprisonment of resonance radiation limits the size of the oriented sample are discussed, and it is suggested that the proposed technique will allow the alignment of larger samples than is possible by the usual method of optical pumping.
A new method has been designed for obtaining the hyperfine optical pumping of Na vapor. The pumping lamp is placed in a magnetic field and the light emitted in the field direction is used for excitation after crossing a circular polarizer selecting one of the Zeeman lines. This method has two interesting features: (i) The atoms can be pumped either in the lower or in the upper hyperfine sublevel by simply rotating a polarizer by an angle of $\frac{1}{2}\ensuremath{\pi}$; (ii) the hyperfine pumping is obtained using both the D1{D}_{1}D1​ and D2{D}_{2}D2​ resonance lines without filtering losses. The pumping efficiency is good and it has been possible to measure the spin-exchange cross section between sodium atoms by studying the hyperfine relaxation rates as a function of the sodium density. The result is ${\ensuremath{\sigma}}_{\mathrm{ex}}=(1.109\ifmmode\pm\else\textpm\fi{}0.005)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ cm2{\mathrm{cm}}^{2}cm2.