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Physical Review A

Measurement of the Rb87 - Xe129 spin-exchange cross section

C. H. Volk, Taeg M. KWON, J. G. Mark

Published May 1, 1980• 74 citations
Publisher

Abstract

The $^{87}\mathrm{Rb}$-$^{129}\mathrm{Xe}$ spin-exchange cross section has been measured in ${\mathrm{N}}_{2}$ buffered cells as a function of the ${\mathrm{N}}_{2}$ density using an alkali vapor magnetometer technique. The strong dependence of the spin-exchange cross section on the ${\mathrm{N}}_{2}$ density indicates a significant role for the Rb-Xe van der Waals molecule in the spin-exchange process. Our data have allowed us to estimate the Rb-Xe molecular formation rate, which is in reasonable agreement with previous measurements. The extrapolated binary spin-exchange cross-section term is found to be ${\ensuremath{\sigma}}_{\mathrm{ex}}{\overline{V}}^{2}=1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ ${\mathrm{cm}}^{4}$ ${\mathrm{sec}}^{\ensuremath{-}2}$.

DOI
10.1103/physreva.21.1549

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The 87Rb^{87}\mathrm{Rb}87Rb-129Xe^{129}\mathrm{Xe}129Xe spin-exchange cross section has been measured in N2{\mathrm{N}}_{2}N2​ buffered cells as a function of the N2{\mathrm{N}}_{2}N2​ density using an alkali vapor magnetometer technique. The strong dependence of the spin-exchange cross section on the N2{\mathrm{N}}_{2}N2​ density indicates a significant role for the Rb-Xe van der Waals molecule in the spin-exchange process. Our data have allowed us to estimate the Rb-Xe molecular formation rate, which is in reasonable agreement with previous measurements. The extrapolated binary spin-exchange cross-section term is found to be \ensuremathσexV‾2=1.5\ifmmode×\else\texttimes\fi10\ensuremath−10{\ensuremath{\sigma}}_{\mathrm{ex}}{\overline{V}}^{2}=1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\ensuremathσex​V2=1.5\ifmmode×\else\texttimes\fi10\ensuremath−10 cm4{\mathrm{cm}}^{4}cm4 sec\ensuremath−2{\mathrm{sec}}^{\ensuremath{-}2}sec\ensuremath−2.
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.
By analyzing the measured spin-relaxation transients of 129Xe^{129}\mathrm{Xe}129Xe nuclear spins and alkali-metal-atomic spins in mixtures of alkali-metal-atom vapors, Xe gas, and larger amounts of N2{\mathrm{N}}_{2}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{p}_{0}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.
By directly observing the nuclear polarization of 129Xe^{129}\mathrm{Xe}129Xe, the efficiency $\ensuremath{\eta}$ of spin exchange between optically pumped Rb spins and 129Xe^{129}\mathrm{Xe}129Xe nuclei has been measured. It is found that $\frac{1}{\ensuremath{\eta}}=23\ifmmode\pm\else\textpm\fi{}4$ rubidium D1{D}_{1}D1​ resonance-line photons are required to polarize a 129Xe^{129}\mathrm{Xe}129Xe nucleus when long-lived van der Waals molecules are unimportant. The binary spin-exchange cross section deduced from our measurements is ${\ensuremath{\sigma}}_{\mathrm{ex}}=(7.3\ifmmode\pm\else\textpm\fi{}1.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}$ cm2{\mathrm{cm}}^{2}cm2.
We propose a fast and accurate method for 129 X e − R b spin-exchange rate determination based on an atomic magnetometer at low magnetic field and low pressure. The 129 X e − R b spin-exchange rate can be estimated via the product of the maximum transverse spin polarization of 129 X e nuclei and the corresponding Rabi frequency. The velocity-averaged binary spin-exchange cross section and the rate constant for spin-exchange in 129 X e − R b van der Waals complexes are measured to be ⟨ σ ν ⟩ S E = ( 1.26 ± 0.13 ) × 10 − 16 c m 3 / s and κ 4 H e = 1326 ± ( 47 ) s − 1 in 4 H e buffer gas, respectively. With respect to the method based on the longitudinal relaxation rate, the method presented in this work can greatly facilitate the measurement process, determining the spin-exchange rate in several minutes. Moreover, this method is more suitable for vapor cells with long longitudinal relaxation time.
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.
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.