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

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 × ${10}^{-10\mathrm{}}$ $p^{-1\mathrm{}}$ 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 ${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$: $\sigma (\mathrm{Rb}-\mathrm{He})=3.1\mathrm{}$ $\sigma (\mathrm{Rb}-\mathrm{Ne})=19\mathrm{}$ $\sigma (\mathrm{Rb}-\mathrm{Ar})=630\mathrm{}$ $\sigma (\mathrm{Rb}-{\mathrm{N}}_2)=83\mathrm{}$. 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°K): $D_0(\mathrm{Rb}-\mathrm{He})=0.42\mathrm{}$, $D_0(\mathrm{Rb}-\mathrm{Ne})=0.235\mathrm{}$, $D_0(\mathrm{Rb}-\mathrm{Ar})=0.16\mathrm{}$, $D_0(\mathrm{Rb}-{\mathrm{N}}_2)=0.16\mathrm{}$. 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 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ for Rb-He, and 5.9 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ 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.