We present measurements of spin-relaxation of Rb atoms in
He(98 mixtures similar to those used in magnetic resonance imaging polarizers. The pressure dependence allows us to separate out contributions from Rb-Xe van der Waals molecules and binary collisions. For the first time, we observe the predicted increase in the molecular contribution at high pressure. Our data suggest that the deduced molecular breakup rate has a strong temperature dependence.
By analyzing the measured spin-relaxation transients of
129Xe nuclear spins and alkali-metal-atomic spins in mixtures of alkali-metal-atom vapors, Xe gas, and larger amounts of
N2 gas, we have determined the three-body formation rates and the spin-transfer probabilities for alkali- metal--noble-gas van der Waals molecules. Three parameters, in addition to the spin quantum numbers of the alkali-metal and noble-gas nuclei, are needed to predict the spin-transfer rates. These parameters, which we have determined from experimental measurements, are x=\ensuremath{\gamma}N/\ensuremath{\alpha}, the ratio of the spin-rotation interaction \ensuremath{\gamma}N to the spin-exchange interaction \ensuremath{\alpha};
p0, the third-body pressure for which the molecular breakup rate ${\ensuremath{\tau}}^{\mathrm{\ensuremath{-}}1}$ is equal to the spin-rotation frequency \ensuremath{\gamma}N/h; and Z, the three-body rate constant for forming van der Waals molecules.
Abstract The relative spin-exchange efficiencies of Cs and Rb remain an important question in the field of hyperpolarized 129 Xe; however, direct experimental comparisons are still limited. We present measurements of the spin-exchange efficiency for 129 Xe–Cs/Rb in 129 Xe: N 2 = 1 : 50 mixtures. Under low-magnetic-field and short-lifetimes van der Waals molecular regime, we determine the spin-exchange efficiencies of 129 Xe–Cs/Rb by measuring the spin-exchange rate coefficient for 129 Xe–Cs/Rb and the Xe-induced spin-relaxation rate of Cs/Rb. Under identical gas composition and pressure conditions, the spin-exchange efficiency of Cs is approximately 1.2–1.5 times higher than Rb. We also derive an expression that enables the prediction of the spin-exchange efficiency for 129 Xe–Cs/Rb in arbitrary 129 Xe– N 2 mixtures.
Spin relaxation of optically pumped
Rb85 and
Rb87 atoms colliding with light-noble-gas atoms is shown to be strongly influenced by an anomalous relaxation process which we attribute to the formation and destruction of bound and quasibound Van der Waals molecules. The anomalous relaxation rate has been measured and analyzed in Ne, complementing an earlier study in He. Relative rates of formation of complexes in two- and three-body collisions have been determined. The correlation time for the quasimolecular interaction in Rb-Ne has been found to be 6.7 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ ${p}^{\ensuremath{-}1}$ sec, where
p is the Ne pressure in Torr. Nuclear-spin-independent cross sections for the relaxation of
〈Sz〉 in sudden binary collisions of Rb atoms with noble-gas atoms have been measured to be (units of ${10}^{\ensuremath{-}24}$
cm2: $\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}\mathrm{He})=3.1$ $\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}\mathrm{Ne})=19$ $\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}\mathrm{Ar})=630$ $\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}{\mathrm{N}}_{2})=83$. Relaxation in sudden binary collisions is shown to follow the theoretically expected nuclear spin dynamics: two relaxation rates in the ratios 8:1 for
Rb87 and 18:1 for
Rb85 have been measured. Diffusion coefficients of Rb in the various buffer gases have been measured to be (units of
cm2/ sec, at 305\ifmmode^\circ\else\textdegree\fi{}K): ${D}_{0}(\mathrm{Rb}\ensuremath{-}\mathrm{He})=0.42$, ${D}_{0}(\mathrm{Rb}\ensuremath{-}\mathrm{Ne})=0.235$, ${D}_{0}(\mathrm{Rb}\ensuremath{-}\mathrm{Ar})=0.16$, ${D}_{0}(\mathrm{Rb}\ensuremath{-}{\mathrm{N}}_{2})=0.16$. Nuclear-spin-independent cross sections for the relaxation of
〈Jz〉 in the
52P21 state of Rb have been measured to be 3.4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$
cm2 for Rb-He, and 5.9 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$
cm2 for Rb-Ne. Anomalies, disagreements, and puzzles occurring in earlier measurements of ground-state relaxation in Rb are shown to be largely resolved when considered in the light of the new results.
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.