Cross Sections for Excited-State Mixing in Cesium-Noble-Gas Collisions fromD2Optical Pumping

Cross sections for random collisional reorientation of J within the sublevels of the $6{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ excited state of cesium have been measured using the $D_2$ optical-pumping technique. The measured cross sections ${{\sigma }_M}^2$ are: Cs-He (3.2±0.6)×${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$, Cs-Ne (2.9±0.6)×${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$, Cs-Ar (6.0±1.2)×${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$, Cs-Kr (9.2±1.9)×${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$, Cs-Xe (11.3±2.3)×${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$. Simplified equations for ${\mathrm{D}}_1$ and ${\mathrm{D}}_2$ pumping with zero, partial, and complete excited-state mixing are derived for a hypothetical alkali atom with zero nuclear spin. The calculations are extended to cesium atoms with nuclear spin $\frac{7}{2}$ pumped by ${\sigma }^{+}$ ${\mathrm{D}}_2$ radiation. The equilibrium value ${〈S_z〉}_0$ and the time dependence $〈S_z\mathrm{}\left(t\right)\mathrm{}〉$ of the electronic spin polarization are computed for various degrees of excited-state mixing. $〈S_z\mathrm{}\left(t\right)\mathrm{}〉$ is found to be nearly single-exponential in the zero- and the complete-mixing cases, but shows a distinct double-exponential behavior in the intermediate region. ${〈S_z〉}_0$ is shown to be dependent on the degree of mixing, and to change sign as the degree of mixing increases. Measurements were performed with a new transmission technique by passing a separate ${\sigma }^{+}$-polarized ${\mathrm{D}}_2$ pumping beam and an alternately ${\sigma }^{+}$- and ${\sigma }^{-}$-polarized ${\mathrm{D}}_1$...