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The Journal of Chemical Physics

Optical orientation effects on the atom-dimer equilibrium composition of lithium vapor

Chun He, Robert A. Bernheim

Published Sep 1, 1993• 2 citations
Publisher

Abstract

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.

DOI
10.1063/1.466120

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Physical Review A•May 1, 1998•2 citations
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.
The shift in the equilibrium distribution between reactants and products in the vapor phase chemical reaction 26Li■6Li2 that is caused by the optical orientation of atomic Li spin angular momenta is shown to be controlled by the degree of atomic orientation. The resolved rotational structure of the 6Li2 electronic spectrum observed by laser induced fluorescence reveals that the major part of the shift occurs for the odd molecular rotational levels associated with the para-nuclear spin variety of the homonuclear 6Li2 molecules. In this respect the results are the same as those previously found for 7Li2 where the nuclei are fermions instead of bosons and the para-nuclear spin symmetry is associated with even-J molecular rotational levels. For He buffer gas pressures of between 50 and 200 Torr and vapor temperatures between 550 and 600 K, over half of the para-molecules could be destroyed using only a few milliwatts of optical pumping power at the lithium D1 wavelength.
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.
The effect of optically pumped atomic spin orientation on the atom–dimer equilibrium in cesium vapor has been observed. Such a shift in equilibrium was predicted over two decades ago and is due to the redistribution of spin-determined bonding and antibonding collision trajectories. Orientation of the total spin angular momentum of 6 2S1/2 ground state Cs atoms in the vapor phase was produced by optical pumping with a tunable cw dye laser. A decrease in the equilibrium molecular density resulted and was measured using laser induced fluorescence from the B 1Πu state. The shift in equilibrium is a function of the degree of atomic orientation and could be observed in the temperature range 383–421 K for cesium vapor in a glass cell which also contained a He–N2 buffer gas mixture with a composition ratio of 9:1.
Optical orientation of Na atoms in a metal vapor with circularly polarized D1{D}_{1}D1​ 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 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.
Ortho-para separations of 7Li2{}^{7}{\mathrm{Li}}_{2}7Li2​ and 6Li2{}^{6}{\mathrm{Li}}_{2}6Li2​ molecules were achieved by using a monochromatic laser beam to align atoms into a single nuclear spin state. This approach is unlike the cryogenic cooling technique, which has been successful only on very light molecules (such as H2{\mathrm{H}}_{2}H2​ and D2{\mathrm{D}}_{2}D2​), which have large energy gaps between the lowest rotational states. The atoms were ``pumped into'' a single hyperfine Zeeman state with defined electron and nuclear spin orientations. The well-resolved rotational structures of the lithium molecules, as shown by laser-induced fluorescence spectra, revealed the enrichment of ortho-components over para-components. Using laser radiation with a power density of $\ensuremath{\sim}10{\mathrm{m}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ can increase the amount of ortho-components by a factor of 2 for 7Li2{}^{7}{\mathrm{Li}}_{2}7Li2​ vapor and by a factor of 3 for 6Li2{}^{6}{\mathrm{Li}}_{2}6Li2​ vapor on a time scale of milliseconds. This paper proposes a model to account for the experimental observations. These increases in the concentrations of ortho-components to para-components are explained by a combined effect of (i) a decrease in molecular density due to the electron spin orientation and (ii) a transfer of nuclear spin momenta from oriented atoms to molecules. The exchange reaction, ${\mathrm{Li}}^{\ensuremath{'}}{+\mathrm{L}\mathrm{i}}_{2}{\ensuremath{\rightarrow}\mathrm{Li}}_{3}{\ensuremath{\rightarrow}\mathrm{Li}}^{\ensuremath{'}}\mathrm{L}\mathrm{i}+\mathrm{L}\mathrm{i},$ appears to transfer the spin orientation from atoms to molecules. A substantial number of ortho-para separations can be achieved through multiple exchange collisions. Applications of this technique to other homonuclear diatomic molecules and polyatomic molecules with nuclear exchange symmetry are predicted.