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Physical Review A

Spin relaxation of rubidium atoms in sudden and quasimolecular collisions with light-noble-gas atoms

F. A. Franz, C. H. Volk

Published Nov 1, 1976• 55 citations
Publisher

Abstract

Spin relaxation of optically pumped ${\mathrm{Rb}}^{85}$ and ${\mathrm{Rb}}^{87}$ 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 $〈{S}_{z}〉$ in sudden binary collisions of Rb atoms with noble-gas atoms have been measured to be (units of ${10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$: $\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}\mathrm{He})=3.1$ $\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}\mathrm{Ne})=19$ $\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}\mathrm{Ar})=630$ $\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}{\mathrm{N}}_{2})=83$. Relaxation in sudden binary collisions is shown to follow the theoretically expected nuclear spin dynamics: two relaxation rates in the ratios 8:1 for ${\mathrm{Rb}}^{87}$ and 18:1 for ${\mathrm{Rb}}^{85}$ have been measured. Diffusion coefficients of Rb in the various buffer gases have been measured to be (units of ${\mathrm{cm}}^{2}$/ 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 $〈{J}_{z}〉$ in the $5^{2}P_{\frac{1}{2}}$ state of Rb have been measured to be 3.4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ for Rb-He, and 5.9 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ 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.

DOI
10.1103/physreva.14.1711

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Spin relaxation of optically pumped Rb85{\mathrm{Rb}}^{85}Rb85 and Rb87{\mathrm{Rb}}^{87}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{10}^{\ensuremath{-}10}10\ensuremath−10 p\ensuremath−1{p}^{\ensuremath{-}1}p\ensuremath−1 sec, where ppp is the Ne pressure in Torr. Nuclear-spin-independent cross sections for the relaxation of 〈Sz〉〈{S}_{z}〉〈Sz​〉 in sudden binary collisions of Rb atoms with noble-gas atoms have been measured to be (units of 10\ensuremath−24{10}^{\ensuremath{-}24}10\ensuremath−24 cm2{\mathrm{cm}}^{2}cm2: \ensuremathσ(Rb\ensuremath−He)=3.1\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}\mathrm{He})=3.1\ensuremathσ(Rb\ensuremath−He)=3.1 \ensuremathσ(Rb\ensuremath−Ne)=19\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}\mathrm{Ne})=19\ensuremathσ(Rb\ensuremath−Ne)=19 \ensuremathσ(Rb\ensuremath−Ar)=630\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}\mathrm{Ar})=630\ensuremathσ(Rb\ensuremath−Ar)=630 \ensuremathσ(Rb\ensuremath−N2)=83\ensuremath{\sigma}(\mathrm{Rb}\ensuremath{-}{\mathrm{N}}_{2})=83\ensuremathσ(Rb\ensuremath−N2​)=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{\mathrm{Rb}}^{87}Rb87 and 18:1 for Rb85{\mathrm{Rb}}^{85}Rb85 have been measured. Diffusion coefficients of Rb in the various buffer gases have been measured to be (units of cm2{\mathrm{cm}}^{2}cm2/ sec, at 305\ifmmode^\circ\else\textdegree\fi{}K): D0(Rb\ensuremath−He)=0.42{D}_{0}(\mathrm{Rb}\ensuremath{-}\mathrm{He})=0.42D0​(Rb\ensuremath−He)=0.42, D0(Rb\ensuremath−Ne)=0.235{D}_{0}(\mathrm{Rb}\ensuremath{-}\mathrm{Ne})=0.235D0​(Rb\ensuremath−Ne)=0.235, D0(Rb\ensuremath−Ar)=0.16{D}_{0}(\mathrm{Rb}\ensuremath{-}\mathrm{Ar})=0.16D0​(Rb\ensuremath−Ar)=0.16, D0(Rb\ensuremath−N2)=0.16{D}_{0}(\mathrm{Rb}\ensuremath{-}{\mathrm{N}}_{2})=0.16D0​(Rb\ensuremath−N2​)=0.16. Nuclear-spin-independent cross sections for the relaxation of 〈Jz〉〈{J}_{z}〉〈Jz​〉 in the 52P125^{2}P_{\frac{1}{2}}52P21​​ state of Rb have been measured to be 3.4 \ifmmode\times\else\texttimes\fi{} 10\ensuremath−16{10}^{\ensuremath{-}16}10\ensuremath−16 cm2{\mathrm{cm}}^{2}cm2 for Rb-He, and 5.9 \ifmmode\times\else\texttimes\fi{} 10\ensuremath−16{10}^{\ensuremath{-}16}10\ensuremath−16 cm2{\mathrm{cm}}^{2}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.
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.
The spin relaxation of optically pumped rubidium vapor induced by rubidium-rare-gas atomic collisions has been investigated in an ultraclean environment. The experiment was performed under conditions such that the partial pressure of nonrare-gas impurities was less than 1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ Torr, and that the time between Rb-Rb spin exchange collisions was short compared to the spin relaxation time. The cross sections (in cm2{\mathrm{cm}}^{2}cm2) for disorientation of rubidium were found to be: He, 3.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}25}$; Ne, 3.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}24}$; Ar, 1.1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}22}$; Kr, 7.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}21}$; Xe, 1.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$. The fact that these cross sections are considerably smaller than previously reported values is attributed primarily to the low degree of impurity contamination in the present work. When compared with already existing values for cesium and sodium, the disorientation cross sections of these three alkali metals are seen to have a remarkably similar dependence on buffer-gas atomic number. The new experimental values are also compared with recent theoretical calculations.
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