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AIP conference proceedings

Spin exchange optical pumping to produce large amounts of polarized nuclei

Will Happer, E. Miron, R. J. Knize, J. L. Cecchi

Published Jan 1, 1984• 4 citations
Publisher

Abstract

We have discussed in detail how spin exchange collisions can transfer momentum from an alkali vapor, which can be easily polarized by optical pumping. For hydrogen atoms the spin exchange rates can be very fast and the alkali vapor can be only a small trace contamination of the atomic hydrogen gas. (AIP)
DOI
10.1063/1.34600

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We report investigations of spin exchange between $^{21}\mathrm{Ne}$ nuclei and optically pumped Rb vapor. Polarization of 30% ($〈{K}_{z}〉=0.46$) of 2\ifmmode\times\else\texttimes\fi{}${10}^{19}$ atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ has been achieved with a vapor pressure of 3\ifmmode\times\else\texttimes\fi{}${10}^{14}$ atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ of Rb. This high density of alkali metal can be efficiently optically pumped only when light trapping is suppressed by inclusion of at least 100 Torr of ${\mathrm{N}}_{2}$. In addition, we have made the first accurate measurement of the binary-collision spin-exchange rate: $〈{\ensuremath{\sigma}}_{\mathrm{ex}}v〉=(4.66\ifmmode\pm\else\textpm\fi{}0.28)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{3}$-${\mathrm{s}}^{\ensuremath{-}1}$. Extension of these techniques to $^{3}\mathrm{He}$ has yielded 5% polarization of 2\ifmmode\times\else\texttimes\fi{}${10}^{19}$ atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ in a preliminary experiment.

Physical Review Letters•Sep 2, 1985•38 citations
Over the four year period covered by this grant, we have carried out extensive research involving polarized atoms and nuclei. Using laser optical pumping, high polarizations can be produced in alkali-metal vapors. From this starting point, angular momentum can be transferred to other atoms and nuclei through several different types of spin-exchange collisions. This powerful technique enables us to study a wide variety of spin interactions and spin related phenomena. Among the systems we work with are polarized nobel gases, which display extremely long spin-relaxation times that vary from several minutes to many weeks. The use of optical pumping and spin exchange has grown enormously over the past four years.
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.
We have produced highly spin-polarized atomic hydrogen by spin-exchange optical pumping. A tunable ring dye laser is used to polarize rubidium atoms by optical pumping. The cell containing the rubidium vapor is coated with paraffin in order to reduce spin relaxation due to wall collisions. Hydrogen gas is dissociated in an inductive discharge and flows continuously through the cell, in which the hydrogen atoms are polarized by spin-exchange collisions with the polarized rubidium atoms. Atomic-hydrogen polarization as high as 2〈Jz{\mathit{J}}_{\mathit{z}}Jz​〉H{\mathrm{〉}}_{\mathrm{H}}〉H​=0.72(6) has been observed, which is the highest polarization yet produced by this method. However, the rubidium polarization may be limited to this value due to radiation trapping at higher rubidium densities. The spin-relaxation rate of atomic hydrogen on a paraffin-coated cell is also measured and corresponds to about 7600 wall bounces between wall relaxation.
The theory of spin exchange between optically pumped alkali-metal atoms and noble-gas nuclei is presented. Spin exchange with heavy noble gases is dominated by interactions in long-lived van der Waals molecules. The main spin interactions are assumed to be the spin-rotation interactions $\ensuremath{\gamma}\stackrel{\ensuremath{\rightarrow}}{\mathrm{N}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}$ between the rotational angular momentum $\stackrel{\ensuremath{\rightarrow}}{\mathrm{N}}$ of the alkali-metal---noble-gas pair and the electron spin $\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}$ of the alkali-metal atom, and the contact hyperfine interaction $\ensuremath{\alpha}\stackrel{\ensuremath{\rightarrow}}{\mathrm{K}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}$ between the nuclear spin $\stackrel{\ensuremath{\rightarrow}}{\mathrm{K}}$ of the noble-gas atom and the electron spin $\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}$. Arbitrary values for KKK and for the nuclear spin III of the alkali-metal atom are assumed. Precise formal expressions for spin transfer coefficients are given along with convenient approximations based on a perturbation expansion in powers of ${(\frac{\ensuremath{\alpha}}{\ensuremath{\gamma}N})}^{2}$, a quantity which has been shown to be small by experiment.
We report investigations of spin exchange between 21Ne^{21}\mathrm{Ne}21Ne nuclei and optically pumped Rb vapor. Polarization of 30% (〈Kz〉=0.46〈{K}_{z}〉=0.46〈Kz​〉=0.46) of 2\ifmmode\times\else\texttimes\fi{}1019{10}^{19}1019 atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ has been achieved with a vapor pressure of 3\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ of Rb. This high density of alkali metal can be efficiently optically pumped only when light trapping is suppressed by inclusion of at least 100 Torr of N2{\mathrm{N}}_{2}N2​. In addition, we have made the first accurate measurement of the binary-collision spin-exchange rate: $〈{\ensuremath{\sigma}}_{\mathrm{ex}}v〉=(4.66\ifmmode\pm\else\textpm\fi{}0.28)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ cm3{\mathrm{cm}}^{3}cm3-${\mathrm{s}}^{\ensuremath{-}1}$. Extension of these techniques to 3He^{3}\mathrm{He}3He has yielded 5% polarization of 2\ifmmode\times\else\texttimes\fi{}1019{10}^{19}1019 atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ in a preliminary experiment.