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Physical Review A

Rb-Xe spin relaxation in dilute Xe mixtures

Ian A. Nelson, Thad G Walker

Published Dec 14, 2001• 54 citations
Publisher

Abstract

We present measurements of spin-relaxation of Rb atoms in $\mathrm{He}(98%)\ensuremath{-}{\mathrm{N}}_{2}(1%)\ensuremath{-}\mathrm{Xe}(1%)$ 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.

DOI
10.1103/physreva.65.012712

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We present measurements of spin-relaxation of Rb atoms in He(98\mathrm{He}(98%)\ensuremath{-}{\mathrm{N}}_{2}(1%)\ensuremath{-}\mathrm{Xe}(1%)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.
We have studied the spin relaxation of 129Xe^{129}\mathrm{Xe}129Xe nuclei due to collisions with Rb atoms at Xe pressures of 245--1817 Torr. Our results can be characterized by two parameters, the Rb${\mathrm{\ensuremath{-}}}^{129}$Xe velocity-averaged binary spin-exchange cross section 〈\ensuremath{\sigma}v〉 and a rate ${\ensuremath{\gamma}}_{\mathit{M}}$ that characterizes spin relaxation due to van der Waals molecules. Our results complement earlier studies performed at Xe pressures of about 1 Torr and N2{\mathrm{N}}_{2}N2​ pressures of 10--100 Torr. This work is useful for predicting spin-exchange rates between polarized Rb atoms and 129Xe^{129}\mathrm{Xe}129Xe nuclei.
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.
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.
The longitudinal relaxation rate of 129Xe^{129}\mathrm{Xe}129Xe nuclear spins slows down substantially in an external magnetic field of about 100 G. From an analysis of the magnetic slowing down and of the dependence of the zero-field relaxation rates on the third-body pressure, the authors have deduced the magnitudes of the spin-rotation and spin-exchange interactions in the Rb129{\mathrm{Rb}}^{129}Rb129Xe molecule, as well as the formation and breakup rates of RbXe in N2{\mathrm{N}}_{2}N2​ gas and the dissociation constant of RbXe. The magnetic slowing down shows directly that binary collisions contribute no more than a few percent to the relaxation rates.
The production of hyperpolarized Xe via spin exchange with optically pumped Rb atoms was studied at 47 000 G under high-pressure conditions. From variable pressure and temperature studies, the spin exchange efficiency was found to be pressure dependent and lower than comparable low-field measurements. Spin exchange due to short binary collisions is expected to be pressure independent, and the cross section was measured to be 〈σν〉SE=6.0 (±0.1)×10−17 cm3 s−1. The pressure dependent component was attributed to the formation of Rb–Xe van der Waals (vdW) complexes. The rate constant for spin exchange in Rb–Xe vdW complexes was measured to be κ=1400 (±100) s−1 in He buffer gas. This value is comparable, though somewhat smaller than vdW rates measured previously for other buffer gases. While it is well known that spin exchange occurs readily in vdW complexes when B0<200 G, vdW spin exchange at fields >10 000 G is not accounted for in current theories. Modulation of the hyperfine interaction during the vdW complex lifetime is proposed to account for these observations. Finally, relaxation by the walls of the pumping cell was found to increase with buffer gas pressure. These factors impact the equilibrium Xe polarization obtainable with high-field optical pumping.