Afterglow Studies in Helium-Cesium Mixtures

The time dependencies of the ${\mathrm{He}}^{+}$, $\mathrm{He}_2^{}{}_{}{}^{+}$, and ${\mathrm{Cs}}^{+}$ ion densities have been measured during the decay period of a plasma produced in helium containing from (${10}^{-6\mathrm{}}$-${10}^{-2\mathrm{}}$)% cesium. Analyses of the ion decay rates as a function of helium pressure indicate that ${\mathrm{Cs}}^{+}$ is produced in the afterglow by collisions between neutral ground-state cesium atoms and ${\mathrm{He}}^{+}$, $\mathrm{He}_2^{}{}_{}{}^{+}$, and $\mathrm{He}\left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S\right)$. The values of the rate constants measured at a gas temperature of $T_g=415\mathrm{}$ °K for these reactions are ${\mathrm{k}}_2$(${\mathrm{He}}^{+}$)=3×${10}^{-10\mathrm{}}$ ${\mathrm{cm}}^3$/sec, ${\mathrm{k}}_3\left(\mathrm{He}2{}_{}{}^3{}_{}{}^{}S\right)=1\mathrm{}\times {10}^{-9\mathrm{}}$ ${\mathrm{cm}}^3$/sec, and ${\mathrm{k}}_4$($\mathrm{He}_2^{}{}_{}{}^{+}$)=2×${10}^{-9\mathrm{}}$ ${\mathrm{cm}}^3$/sec. The reduced mobility of ${\mathrm{Cs}}^{+}$ in helium at 415 °K was measured to be (15.5±1) ${\mathrm{cm}}^2$/V sec.