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Physical Review A

Role of Na-Xe molecules in spin relaxation of optically pumped Na in Xe gas

Natarajan D. Bhaskar, M. Hou, Martin Ligare, B Suleman, W. Happer

Published Dec 1, 1980• 16 citations
Publisher

Abstract

A mixture of dense sodium vapor (number density [Na] \ensuremath{\sim} ${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$), and buffer gases helium, xenon, and nitrogen is optically pumped to a high degree of spin polarization using a single-frequency single-mode tunable dye laser. The relaxation in the "dark" of the variables $〈{S}_{z}〉$ and $〈\stackrel{\ensuremath{\rightarrow}}{I}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{S}〉$ is studied as a function of [Na] and buffergas pressures. For lower helium pressures (46-torr helium and 2-torr Xe) the dominant contribution to the relaxation of $〈{S}_{z}〉$ in the presence of Xe comes from the "sticking" three-body collisions with He acting as the third body. This is further confirmed by studying the longitudinal magnetic field dependence of the spin-relaxation rate. At higher helium pressures (He 700 torr, and Xe 2 torr) the relaxation of $〈{S}_{z}〉$ in the presence of Xe is mainly due to the "sudden" binary collisions between the Na and Xe, and the relaxation rate of $〈{S}_{z}〉$ is independent of the longitudinal magnetic field. Significant hyperfine pumping is achieved by proper tuning of the laser. The temperature dependence of the relaxation of $〈\stackrel{\ensuremath{\rightarrow}}{I}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{S}〉$ and $〈{S}_{z}〉$ is also discussed.

DOI
10.1103/physreva.22.2710

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We report the first detection of the effect of Na–Na spin exchange collisions on spin relaxation of optically pumped Na in Xe gas by studying the relaxation in the "dark" of the variable ⟨ S z ⟩ in a mixture of dense sodium vapor (number density [Na] ~ 10 13 cm −3 ), and buffer gas helium, xenon and nitrogen as a function of [Na] and xenon pressure.

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Stephan Appelt, Andrei Ben-Amar Baranga, Christopher Erickson +3 more

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A nuclear spin-based129Xe and131Xe using optical polarization

Binquan Zhou, Linlin Chen, Guanqun Lei +3 more

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Oct 8, 2015•0 citations
A mixture of dense sodium vapor (number density [Na] \ensuremath{\sim} 1013{10}^{13}1013 cm\ensuremath−3{\mathrm{cm}}^{\ensuremath{-}3}cm\ensuremath−3), and buffer gases helium, xenon, and nitrogen is optically pumped to a high degree of spin polarization using a single-frequency single-mode tunable dye laser. The relaxation in the "dark" of the variables 〈Sz〉〈{S}_{z}〉〈Sz​〉 and 〈I\ensuremath→\ifmmode⋅\else\textperiodcentered\fiS\ensuremath→〉〈\stackrel{\ensuremath{\rightarrow}}{I}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{S}〉〈I\ensuremath→​\ifmmode⋅\else\textperiodcentered\fiS\ensuremath→​〉 is studied as a function of [Na] and buffergas pressures. For lower helium pressures (46-torr helium and 2-torr Xe) the dominant contribution to the relaxation of 〈Sz〉〈{S}_{z}〉〈Sz​〉 in the presence of Xe comes from the "sticking" three-body collisions with He acting as the third body. This is further confirmed by studying the longitudinal magnetic field dependence of the spin-relaxation rate. At higher helium pressures (He 700 torr, and Xe 2 torr) the relaxation of 〈Sz〉〈{S}_{z}〉〈Sz​〉 in the presence of Xe is mainly due to the "sudden" binary collisions between the Na and Xe, and the relaxation rate of 〈Sz〉〈{S}_{z}〉〈Sz​〉 is independent of the longitudinal magnetic field. Significant hyperfine pumping is achieved by proper tuning of the laser. The temperature dependence of the relaxation of 〈I\ensuremath→\ifmmode⋅\else\textperiodcentered\fiS\ensuremath→〉〈\stackrel{\ensuremath{\rightarrow}}{I}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{S}〉〈I\ensuremath→​\ifmmode⋅\else\textperiodcentered\fiS\ensuremath→​〉 and 〈Sz〉〈{S}_{z}〉〈Sz​〉 is also discussed.
We present a comprehensive theory of nuclear spin polarization of 3He{}^{3}\mathrm{He}3He and 129Xe{}^{129}\mathrm{Xe}129Xe gases by spin-exchange collisions with optically pumped alkali-metal vapors. The most important physical processes considered are (1) spin-conserving spin-exchange collisions between like or unlike alkali-metal atoms; (2) spin-destroying collisions of the alkali-metal atoms with each other and with buffer-gas atoms; (3) electron-nuclear spin-exchange collisions between alkali-metal atoms and 3He{}^{3}\mathrm{He}3He or 129Xe{}^{129}\mathrm{Xe}129Xe atoms; (4) spin interactions in van der Waals molecules consisting of a Xe atom bound to an alkali-metal atom; (5) optical pumping by laser photons; (6) spatial diffusion. The static magnetic field is assumed to be small enough that the nuclear spin of the alkali-metal atom is well coupled to the electron spin and the total spin is very nearly a good quantum number. Conditions appropriate for the production of large quantities of spin-polarized 3He{}^{3}\mathrm{He}3He or 129Xe{}^{129}\mathrm{Xe}129Xe gas are assumed, namely, atmospheres of gas pressure and nearly complete quenching of the optically excited alkali-metal atoms by collisions with N2{\mathrm{N}}_{2}N2​ or H2{\mathrm{H}}_{2}H2​ gas. Some of the more important results of this work are as follows: (1) Most of the pumping and relaxation processes are sudden with respect to the nuclear polarization. Consequently, the steady-state population distribution of alkali-metal atoms is well described by a spin temperature, whether the rate of spin-exchange collisions between alkali-metal atoms is large or small compared to the optical pumping rate or the collisional spin-relaxation rates. (2) The population distributions that characterize the response to sudden changes in the intensity of the pumping light are not described by a spin temperature, except in the limit of very rapid spin exchange. (3) Expressions given for the radio-frequency (rf) resonance linewidths and areas can be used to make reliable estimates of the local spin polarization of the alkali-metal atoms. (4) Diffusion effects for these high-pressure conditions are mainly limited to thin layers at the cell surface and at internal resonant surfaces generated by radio-frequency magnetic fields when the static magnetic field has substantial spatial inhomogeneities. The highly localized effects of diffusion at these surfaces are described with closed-form analytic functions instead of the spatial eigenmode expansions that are appropriate for lower-pressure cells.
The spin relaxation of optically pumped rubidium vapor induced by rubidium-rare-gas atomic collisions has been investigated in an ultraclean environment. The experiment was performed under conditions such that the partial pressure of nonrare-gas impurities was less than 1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ Torr, and that the time between Rb-Rb spin exchange collisions was short compared to the spin relaxation time. The cross sections (in cm2{\mathrm{cm}}^{2}cm2) for disorientation of rubidium were found to be: He, 3.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}25}$; Ne, 3.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}24}$; Ar, 1.1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}22}$; Kr, 7.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}21}$; Xe, 1.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$. The fact that these cross sections are considerably smaller than previously reported values is attributed primarily to the low degree of impurity contamination in the present work. When compared with already existing values for cesium and sodium, the disorientation cross sections of these three alkali metals are seen to have a remarkably similar dependence on buffer-gas atomic number. The new experimental values are also compared with recent theoretical calculations.
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We experimentally study the Rb-Xe spin-exchange optical pumping process. The dependence of the spin-exchange rate on the intensity of the pumping light is measured at different temperatures under low magnetic fields. We demonstrate that the spin-exchange rate will decrease as the Rb spin polarization increases, which agrees well with the theoretical prediction. In our measurement, three spin exchange and spin relaxation mechanisms, namely, the spin exchange rates between Rb and Xe atoms caused by van der Waals molecule (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{vdW}}$) and binary collision (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{bin}}$) processes, and the spin relaxation rate due to the wall collision of Xe atoms (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{w}}$), have comparable magnitudes. These rates are extracted separately from the measured data with different Rb spin polarization at various temperatures. Our work provides a comprehensive confirmation of the physical picture of the spin-exchange optical pumping process.
We present a design for a spin-exchange optical pumping system to produce large quantities of highly polarized 129Xe and 131Xe. Low xenon concentrations in the flowing gas mixture which allow the laser to maintain high Cs polarization. The large spin-exchange rate between Cs and Xe through the long-lived van der Waals molecules at low pressure, combined with a high flow rate, results in large production rates of hyperpolarized xenon. The fast rates make it possible to obtain large nuclear polarizations after several minutes of optical pumping with a laser.At high Xe pressures. According to the theory, the longitudinal spin-elaxation rate 1T1 of Xe in a high-pressure sample containing only Xe and Cs vapor has the simple form is the velocity averaged binary spin-exchange cross section, It is the relaxation rate due to wall collisions and perhaps magnetic field inhomogeneities. Our results complement earlier studies performed at 129Xe pressures of about 20 Torr and 131Xe pressures of about 20 Torr and N2 pressures of 600 Torr . This work is useful for predicting spin-exchange rates between polarized Cs atoms and Xe nuclei.