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arXiv:physics/0009072 · Physical Review Letters

Spin Relaxation Resonances due to the Spin-Axis Interaction in Dense Rubidium and Cesium Vapor

Christopher Erickson, David Levron, W. Happer, Stephen J. Kadlecek, Bien Chann, L. W. Anderson, Thad G Walker

Published Nov 13, 2000• 37 citations
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Abstract

Resonances in the magnetic decoupling curves for the spin relaxation of dense alkali-metal vapors prove that much of the relaxation is due to the spin-axis interaction in triplet dimers. Initial estimates of the spin-axis coupling coefficients for the dimers (likely accurate to a factor of 2) are $|\ensuremath{\lambda}|/h\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}290\mathrm{MHz}$ for Rb; $2500\mathrm{MHz}$ for Cs.

DOI
10.1103/physrevlett.85.4237

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Resonances in the magnetic decoupling curves for the spin relaxation of dense alkali-metal vapors prove that much of the relaxation is due to the spin-axis interaction in triplet dimers. Initial estimates of the spin-axis coupling coefficients for the dimers (likely accurate to a factor of 2) are ∣\ensuremathλ∣/h=290MHz|\ensuremath{\lambda}|/h\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}290\mathrm{MHz}∣\ensuremathλ∣/h=290MHz for Rb; 2500MHz2500\mathrm{MHz}2500MHz for Cs.
We present calculations of spin-relaxation rates of alkali-metal atoms due to the spin-axis interaction acting in binary collisions between the atoms. We show that for the high-temperature conditions of interest here, the spin-relaxation rates calculated with classical-path trajectories are nearly the same as those calculated with the distorted-wave Born approximation. We compare these calculations to recent experiments that used magnetic decoupling to isolate spin relaxation due to binary collisions from that due to the formation of triplet van der Waals molecules. The values of the spin-axis coupling coefficients deduced from measurements of binary collision rates are consistent with those deduced from molecular decoupling experiments, and follow a physically plausible scaling law for the spin-axis coupling coefficients.
We have completed a thorough study of masing action of hyperpolarized He-3 gas in inhomogeneous magnetic fields. The self interaction of the He-3 magnetization has a large effect on the maser gain. We have developed a simple new magnetic resonance method for investigating the interactions of Xe-129 spins with protons in polymers. We have shown that there are novel magnetic resonance transitions in alkali-metal vapors that have vanishing spin-exchange broadening in the limit of high spin polarization. Such transitions could be useful for miniaturized atomic clocks. We have shown that spin-axis interactions in triplet molecules are responsible for much of the spin destruction rate dense alkali-metal vapors.
We discuss the relaxation of a spin-polarized alkali-metal vapor due to the electric-quadrupole interaction in ${}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ dimers, and present a model that accurately describes the measured relaxation rates under a wide variety of conditions. We use the model along with experimental results to deduce the quadrupole interaction strength, atom-dimer chemical-exchange reaction rate, and the rate of reorientation of singlet dimers due to collisions with the buffer gas.
We report the first studies of magnetic decoupling of the spin relaxation of alkali-metal atoms due to binary collisions with buffer gases. When binary collisions are the dominant relaxation mechanism, the relaxation and its magnetic decoupling are well described by the SSS-damping rate ${\ensuremath{\Gamma}}_{\mathrm{SD}}$ due to the spin-rotation interaction $\ensuremath{\gamma}\mathbf{N}\ifmmode \dot{}\else \.{}\fi{}\mathbf{S}$, the spin exchange rate ${\ensuremath{\Gamma}}_{\mathrm{EX}}$ for collisions between alkali atoms, and a new ``Carver rate'' ${\ensuremath{\Gamma}}_{\mathrm{C}}$, due to the pressure-shift interaction $\ensuremath{\delta}A\mathbf{I}\ifmmode \dot{}\else \.{}\fi{}\mathbf{S}$, which can substantially broaden the magnetic decoupling curve while having no influence on the zero-field rates.
By measuring the relaxation rates of the nuclear spin polarization of 129Xe^{129}\mathrm{Xe}129Xe in the presence of alkali-metal vapor at different densities, we have extracted the spin-exchange rates between 129Xe^{129}\mathrm{Xe}129Xe and the three alkali metals K, Rb, and Cs. By studying the alkali-metal--129Xe^{129}\mathrm{Xe}129Xe spin-exchange rates as functions of the cell number density from 0.2 to 0.7 amagat, the binary collision and van der Waals molecular terms are separated, and constants governing both mechanisms are determined. The results from our work can be used to optimize the parameter space for polarizing 129Xe^{129}\mathrm{Xe}129Xe, a promising agent for magnetic resonance imaging and other applications.
We report studies of the magnetic decoupling of Rb spin relaxation in the presence of H2{\mathrm{H}}_{2}H2​ buffer gas at high pressures (> 1.5atm).(>~1.5\mathrm{atm}).(> 1.5atm). At low Rb number densities $(<~5\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{cm}}^{\ensuremath{-}3}),$ we find that the observed magnetic-field dependence of the spin relaxation is almost solely due to binary collisions between Rb atoms and hydrogen molecules, and that it is very well described by three rates: the Rb-Rb spin-exchange rate, the S-damping rate, and the Carver rate associated with the hyperfine pressure-shift interaction in collisions of Rb with H2{\mathrm{H}}_{2}H2​ molecules. At higher Rb densities, the contribution to the relaxation from Rb triplet dimers becomes significant.