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arXiv:1012.0593 · Physical Review Letters

Anisotropic Hyperfine Interactions Limit the Efficiency of Spin-Exchange Optical Pumping of He3 Nuclei

Timur V. Tscherbul, P. Zhang, H. R. Sadeghpour, A. Dalgarno

Published Jul 8, 2011• 18 citations
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Abstract

We use accurate ab initio and quantum scattering calculations to demonstrate that the maximum ³He spin polarization that can be achieved in spin-exchange collisions with potassium (³⁹K) and silver (¹⁰⁷Ag) atoms is limited by the anisotropic hyperfine interaction. We find that spin exchange in Ag-He collisions occurs much faster than in K-He collisions over a wide range of temperatures (10-600 K). Our analysis indicates that measurements of trap loss rates of ²S atoms in the presence of cold ³He gas may be used to probe anisotropic spin-dependent interactions in atom-He collisions.

DOI
10.1103/physrevlett.107.023204

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•Jan 1, 2004•1 citation
We use accurate ab initio and quantum scattering calculations to demonstrate that the maximum ³He spin polarization that can be achieved in spin-exchange collisions with potassium (³⁹K) and silver (¹⁰⁷Ag) atoms is limited by the anisotropic hyperfine interaction. We find that spin exchange in Ag-He collisions occurs much faster than in K-He collisions over a wide range of temperatures (10-600 K). Our analysis indicates that measurements of trap loss rates of ²S atoms in the presence of cold ³He gas may be used to probe anisotropic spin-dependent interactions in atom-He collisions.
We report on extensive experimental measurements of the key rates that determine the efficiency for polarizing the nuclei of 3He{}^{3}\mathrm{He}3He by spin exchange with optically pumped Rb vapor. In agreement with recent theoretical predictions, we find a strong temperature dependence of the electron-spin loss rates due to 3HeRb{}^{3}\mathrm{HeRb}3HeRb collisions. We also find that the maximum possible efficiency for spin-exchange polarization of 3He{}^{3}\mathrm{He}3He by K is 10 times greater than for Rb.
Collisions of atoms and molecules in external electromagnetic fields can depolarize electronic and nuclear spins, leading to decoherence of quantum superposition states. The fundamental microscopic processes giving rise to spin depolarization are spin exchange and spin relaxation. In this work, we present an analysis of spin exchange transitions in collision of alkali metal atoms with 3He. We use accurate ab initio calculations to evaluate the electron spin density of Li-He and the interaction energy of Rb-He. The Fermi contact interaction constants for the complexes of Na, K, and Rb atoms with 3He are extracted from the measured frequency shift enhancement factors using the ab initio interaction potentials. The spin density calculations also yield the hyperfine pressure shift of Li-He.
We present a detailed theoretical analysis of the noble-gas nuclear-spin relaxation due to the anisotropic magnetic-dipole hyperfine interaction between the noble-gas nucleus and alkali-metal valence electron vis \`a vis the already well-understood (spin-conserving) isotropic magnetic-dipole hyperfine interaction in alkali-metal--noble-gas systems. We find that, for all pairs in which the noble gas is not helium, the predicted spin-relaxation rate from the anisotropic interaction does not exceed 2.5% of the rate from the isotropic interaction, thereby not appreciably limiting the maximum noble-gas nuclear polarization attainable via spin-exchange collisions with polarized alkali-metal atoms. For alkali-metal--helium pairs, we predict that the anisotropic interaction has a slightly larger relative effect, perhaps limiting the nuclear polarization to $\ensuremath{\sim}95%$ of the electronic polarization in the Rb-3He{}^{3}\mathrm{He}3He system; however, our confidence in the helium results is limited by a lack of knowledge of the interatomic potentials necessary for the calculation.
We demonstrate spin-exchange optical pumping of 3He^{3}\mathrm{H}\mathrm{e}3He using a ``hybrid'' K-Rb vapor mixture. The Rb atoms absorb light from a standard laser at 795 nm, then collisionally polarize the potassium atoms. Spin-exchange collisions of K and 3He^{3}\mathrm{H}\mathrm{e}3He atoms then transfer the angular momentum to the 3He^{3}\mathrm{H}\mathrm{e}3He with much greater efficiency than $\mathrm{R}\mathrm{b}\mathrm{\text{\ensuremath{-}}}^{3}\mathrm{H}\mathrm{e}$. For a K-rich vapor, the efficiency of the hybrid spin-exchange collisions approaches 1/41/41/4, an order of magnitude greater than achieved by pure Rb pumping. We present the first measurements of actual photon efficiencies (polarized nuclei produced per absorbed photon), and show that a new parasitic absorption process limits the total efficiencies for both hybrid and pure Rb pumping.
We present experimental studies on the spin-exchange optical pumping of 3He using alkali metal potassium vapor. High 3He polarizations are achieved using lower laser power than traditional rubidium-3He pumping studies. In addition, spin-exchange rate coefficient measurements for potassium-3He, rubidium-3He and cesium-3He pairs are reported. By spin-exchange optical pumping with potassium vapor, a high 3He polarization has been achieved in a mid-sized double-chamber glass target cell using only 5 watts of Ti:sapphire laser power. The spin-exchange rate coefficients of potassium-3He, cesium-3He and rubidium-3He pairs are measured and compared. The results are kSE = (4.0+/-0.3)E-20 cm3/sec for the potassium-3He pair, kSE = (6.5+/-0.4)E-20 cm3/sec for the rubidium-3He pair and kSE = (13.6 +/-1.3)E-20 cm3/sec for the cesium-3He pair. The results are consistent with theoretical predictions and confirm that the efficiency for spin-exchange polarization of 3He with potassium is significantly higher than with rubidium or cesium. The results motivate the development of high power diode lasers tuned to the appropriate potassium wavelengths. Applied to the production of polarized 3He as an atomic beam source for colliders, these developments could have significant impact on future studies of neutron spin structure. These studies may also have impact on magnetic resonance imaging using hyperpolarized 3He.