Relaxation experiments performed on optically polarized Rb atoms in a rare gas have been previously reported; their detailed interpretation is given below. It is 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. Two processes of molecule formation are analyzed: binary resonant collisions leading to metastable states and three-body collisions producing actual bound states. A relaxation model valid for any disorientation probability per single Rb-Kr interaction is developed. Aside from clear evidence for the existence of alkali-rare-gas molecules, the success of the theoretical interpretation of the relaxation experiments yields the equilibrium constant $\mathcal{K}=1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}$
cm3/molecule for the reaction Rb+Kr \ensuremath{\rightleftharpoons} Rb-Kr at 300\ifmmode^\circ\else\textdegree\fi{}K, the average lifetime of a Rb-Kr molecule in the gas phase, $\ensuremath{\tau}=0.65\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ sec, at a krypton pressure of one Torr, and the average spin-orbit coupling constant in a Rb-Kr molecule, $\overline{\ensuremath{\gamma}}{h}^{\ensuremath{-}1}=0.63$ MHz. It is also shown that the spin-orbit potential is predominantly short-range.
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.
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 of optically pumped Rb atoms on paraffin-coated walls has been studied using Franzen's sequence (relaxation in the dark) for observables
〈Sz〉 (mean electronic longitudinal polarization), and <S\ifmmode\cdot\else\textperiodcentered\fi{}I> (population difference between the two hfs levels). The physical meaning of optical signals under different conditions is analyzed in some detail, and it is pointed out that different observables relax differently. In the interpretation of our results, extensive use has been made of the theoretical analysis of the relaxation of a spin S (coupled to a spin I in the Rb atom) caused by a weak interaction of the type $_{\ensuremath{\gamma}s}H(t)\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{S}$,
H(t) being a random magnetic field acting on S during the dwell time of a Rb atom on the wall. Experimental results are in excellent agreement with this analysis, if one assumes the existence of two uncorrelated interactions of the above type. The first is the dipole-dipole interaction between S and the nuclear spins K of the protons (or deuterons) of the coating. This interaction has a long correlation time ${\ensuremath{\tau}}_{c1}\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec and dominates the relaxation of
〈Sz〉 in low fields. The second interaction which is independent of spins K, has a short correlation time ${\ensuremath{\tau}}_{c2}\ensuremath{\approx}{10}^{\ensuremath{-}12}$ sec, and dominates the relaxation of <S\ifmmode\cdot\else\textperiodcentered\fi{}I> in low fields, and of
〈Sz〉 in large fields. The values of these correlation times and of the strengths of the two interactions have been measured, first in low fields by combining results on
〈Sz〉 and <S\ifmmode\cdot\else\textperiodcentered\fi{}I> for the two Rb isotopes and for two types of coatings [
(CH2)n and
(CD2)n], and second, by studying the variations of the "pseudo" relaxation time
T1 of
〈Sz〉 for magnetic-field values up to 5000 G. We arrive at a detailed picture of physical adsorption of Rb on paraffin-coated walls, and find 0.1 eV for the adsorption energy. These results are compatible with those obtained for the relaxation of oriented Rb atoms in collisions with buffer-gas molecules.
Relaxation experiments have been performed between 0–200 G on optically polarized Rb atoms in argon, krypton or xenon, at pressures ranging from 0.1 to 20 torr. They clearly show the existence of Rb–Ar, Rb–Kr and Rb–Xe molecules of low binding energy (smaller than kT). Molecular parameters such as lifetime, spin-orbit coupling constant, formation rate, constant of mass action, etc., are measured, or evaluated when they cannot be directly measured. Experimental values of the diffusion coefficient of Rb in the gas (except xenon) and of the disorientation cross sections by usual binary collisions are also obtained.