Cross Sections for Excited-State Mixing in Cesium-Noble-Gas Collisions from D2 Optical Pumping

Abstract

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 ${{\ensuremath{\sigma}}_{M}}^{2}$ are: Cs-He (3.2\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$, Cs-Ne (2.9\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$, Cs-Ar (6.0\ifmmode\pm\else\textpm\fi{}1.2)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$, Cs-Kr (9.2\ifmmode\pm\else\textpm\fi{}1.9)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$, Cs-Xe (11.3\ifmmode\pm\else\textpm\fi{}2.3)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$. 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 ${\ensuremath{\sigma}}^{+}$ ${\mathrm{D}}_{2}$ radiation. The equilibrium value ${〈{S}_{z}〉}_{0}$ and the time dependence $〈{S}_{z}(t)〉$ of the electronic spin polarization are computed for various degrees of excited-state mixing. $〈{S}_{z}(t)〉$ 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 ${\ensuremath{\sigma}}^{+}$-polarized ${\mathrm{D}}_{2}$ pumping beam and an alternately ${\ensuremath{\sigma}}^{+}$- and ${\ensuremath{\sigma}}^{\ensuremath{-}}$-polarized ${\mathrm{D}}_{1}$ testing beam through the cesium absorption cell. The cross sections for collisionally induced mixing were evaluated by measuring the buffer pressure at which ${〈{S}_{z}〉}_{0}$ passed through zero.