Spin relaxation of Rb atoms induced by collisional modification of the hyperfine interaction has been measured for
85Rb and
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 in He, 0.029 ${\mathrm{sec}}^{\ensuremath{-}1}$ ${\mathrm{Torr}}^{\ensuremath{-}1}$ for
85Rb in Ne, and 0.041 ${\mathrm{sec}}^{\ensuremath{-}1}$ ${\mathrm{Torr}}^{\ensuremath{-}1}$ for
85Rb in Ar. The corresponding relaxation rates for
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 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. Anomalous relaxation,
R∗, is present in the same degree at 40 kG as at low magnetic field, lending strong support to earlier attributions of
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.
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.
Relaxation experiments performed on alkali atoms polarized in their ground state by optical pumping in the presence of a rare gas have been previously reported and interpreted. It was shown that the relaxation governed by the spin-orbit interaction is strongly affected by the formation of chemically unstable Rb-Kr molecules bound by van der Waals forces. The previous theoretical analysis, restricted to the longitudinal relaxation, is extended here to relaxation processes involving the transverse components of the electronic polarization and hyperfine "coherences" (elements of the density matrix between different hyperfine states). The effects resulting from the action of a rf field are also examined. The main results of this work, among others, are (a) the prediction of a Zeeman transition-line pressure shift, in some circumstances larger by several orders of magnitude than the one calculated by Herman, and having a very peculiar variation with the rare-gas pressure; (b) a quantitative explanation of the anomalous pressure dependence of the hyperfine transition linewidth discovered by Bender and Cohen; (c) a proposal of a method of detection of rf transitions between states of the alkali---rare-gas molecules by their effect on the relaxation process.
Electronic and nuclear spin relaxation of optically pumped K atoms has been measured in the presence of He and Ne gases. The nuclear-spin-independent cross sections for the destruction of electronic spin polarization in the
42S21 ground state of K are $\ensuremath{\sigma}[\ensuremath{\gamma}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{N}})](\mathrm{K}\ensuremath{-}\mathrm{He})=8.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}$
cm2 and $\ensuremath{\sigma}[\ensuremath{\gamma}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{N}})](\mathrm{K}\ensuremath{-}\mathrm{Ne})=141\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}$
cm2. The diffusion coefficients of K in He and Ne, corrected to 273 K, are ${D}_{0}(\mathrm{K}\ensuremath{-}\mathrm{He})=0.35$
cm2 ${\mathrm{sec}}^{\ensuremath{-}1}$ and ${D}_{0}(\mathrm{K}\ensuremath{-}\mathrm{Ne})=0.19$
cm2 ${\mathrm{sec}}^{\ensuremath{-}1}$. All measurements have an estimated uncertainty of \ifmmode\pm\else\textpm\fi{}10%. The relaxation of K is shown to be strongly affected by an anomalous process which is attributed to the modification of the K hyperfine interaction induced by the formation of K-He and K-Ne van der Waals complexes.
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) 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.