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Physical Review A

Magnetic decoupling of Rb spin relaxation in H2 buffer gas

P. I. Borel, Christopher Erickson, D. K. Walter

Published Dec 20, 2002• 1 citation
Publisher

Abstract

We report studies of the magnetic decoupling of Rb spin relaxation in the presence of ${\mathrm{H}}_{2}$ buffer gas at high pressures $(>~1.5\mathrm{atm}).$ 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 ${\mathrm{H}}_{2}$ molecules. At higher Rb densities, the contribution to the relaxation from Rb triplet dimers becomes significant.

DOI
10.1103/physreva.66.063410

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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×\else\texttimes\fi1013cm\ensuremath−3),(<~5\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{cm}}^{\ensuremath{-}3}),(< 5\ifmmode×\else\texttimes\fi1013cm\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.
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.
Spin relaxation of Rb atoms induced by collisional modification of the hyperfine interaction has been measured for 85Rb^{85}\mathrm{Rb}85Rb and 87Rb^{87}\mathrm{Rb}87Rb in He, Ne, and Ar buffer gases at high magnetic field. The relaxation rates at 40 kG are 0.028 ${\mathrm{sec}}^{\ensuremath{-}1}$ ${\mathrm{Torr}}^{\ensuremath{-}1}$ for 85Rb^{85}\mathrm{Rb}85Rb in He, 0.029 ${\mathrm{sec}}^{\ensuremath{-}1}$ ${\mathrm{Torr}}^{\ensuremath{-}1}$ for 85Rb^{85}\mathrm{Rb}85Rb in Ne, and 0.041 ${\mathrm{sec}}^{\ensuremath{-}1}$ ${\mathrm{Torr}}^{\ensuremath{-}1}$ for 85Rb^{85}\mathrm{Rb}85Rb in Ar. The corresponding relaxation rates for 87Rb^{87}\mathrm{Rb}87Rb are approximately 4.9 times larger, in accord with theory. Cross sections for the binary $\ensuremath{\delta}a(\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{I}})$ collisional interaction are estimated to be $\ensuremath{\sigma}(\ensuremath{\delta}a(\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{I}}))(\mathrm{Rb}\ensuremath{-}\mathrm{He})=2.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ cm2{\mathrm{cm}}^{2}cm2 and $\ensuremath{\sigma}(\ensuremath{\delta}a(\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{I}}))(\mathrm{Rb}\ensuremath{-}\mathrm{Ne})=1.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ cm2{\mathrm{cm}}^{2}cm2. Anomalous relaxation, R∗{R}^{*}R∗, is present in the same degree at 40 kG as at low magnetic field, lending strong support to earlier attributions of R∗{R}^{*}R∗ to the $\ensuremath{\delta}a(\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{I}})$ interaction in Rb-atom-noble-gas-atom van der Waals molecules.
We have measured the binary spin-exchange rate coefficient for collisions between Rb and 129Xe{}^{129}\mathrm{Xe}129Xe atoms at a magnetic field B=9.4TB=9.4\mathrm{T}B=9.4T and in the temperature range $160<T<200\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ The longitudinal spin-relaxation rate 1/T1{1/T}_{1}1/T1​ of 129Xe{}^{129}\mathrm{Xe}129Xe is dominated by ${}^{129}\mathrm{Xe}\ensuremath{-}\mathrm{Rb}$ binary collisions. We measured 1/T1{1/T}_{1}1/T1​ using the methods of conventional nuclear magnetic resonance, with no spin enhancement by optical pumping. At the high magnetic field and large gas pressures used in this work, the contribution of ${}^{129}\mathrm{Xe}\ensuremath{-}\mathrm{Rb}$ molecules to the spin-exchange rate was less than 0.1% of the binary-collision rate. The Rb atomic number density [Rb] was measured directly from the large Faraday rotation of the polarization of near-resonant light passing through the sample cells. The rate coefficient is $\ensuremath{\kappa}{=d(1/T}_{1})/d[\mathrm{Rb}]=(1.75\ifmmode\pm\else\textpm\fi{}0.12)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}{\mathrm{cm}}^{3}/\mathrm{s}.$
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 report calculations of the magnetic decoupling of the nuclear spin relaxation rate of 129Xe{}^{129}\mathrm{Xe}129Xe atoms caused by binary collisions with Rb or Cs atoms in a gas. A semiclassical approximation, using a classical path for the collision and treating the spin interactions quantum mechanically, gives results that differ by no more than 2% from calculations with the distorted-wave Born approximation. Both computational methods show that the relaxation rate decreases substantially for magnetic fields of a few tens of teslas. For ${}^{129}\mathrm{Xe}\ensuremath{-}\mathrm{Rb},$ where recent experimental measurements are available, the calculated rate coefficients at the experimental field of 9.4 T are about 80% of the calculated values at zero field. At 9.4 T, the calculated rate coefficient for ${}^{129}\mathrm{Xe}\ensuremath{-}\mathrm{Rb}$ is about one-half of the experimentally observed value. This discrepancy is probably due to inaccuracies in the van der Waals and spin-dependent potentials used in the calculations.