Atomic Processes in Helium-Krypton and Helium-Xenon Mixtures

The momentum transfer collision frequency of thermal electrons with neutrals in a decaying plasma established in helium-krypton and helium-xenon mixtures of known proportions was measured by microwave interferometer at gas temperatures of ∼200 to 600°K. The energy dependences of the momentum transfer cross sections of electrons with krypton and xenon atoms deduced from these measurements are best represented by $Q_m\mathrm{}\left(u\right)=6.56\mathrm{}\times {10}^{-15\mathrm{}}-2.79\mathrm{}\times {10}^{-14\mathrm{}}{u}^{\frac{1}{2}}+3.14\mathrm{}\times {10}^{-14\mathrm{}}u$ and $1.91\times {10}^{-14\mathrm{}}-8.30\mathrm{}\times {10}^{-14\mathrm{}}{u}^{\frac{1}{2}}+9.40\mathrm{}\times {10}^{-14\mathrm{}}u$ ${\mathrm{cm}}^2$, respectively. Here $u$ is the electron energy in electron volts. Mobilities of ${\mathrm{Kr}}^{+}$ and ${\mathrm{Xe}}^{+}$ in helium and their respective parent gas have also been determined, from the characteristic time constants of the electron density decay measured in the afterglow in the mixtures at low pressures, to be $\mu \left({\mathrm{Kr}}^{+} \mathrm{in}\mathrm{He}\right)=20.2\mathrm{}\pm 1.2$ ${\mathrm{cm}}^2$/V-sec, $\mu \left({\mathrm{Kr}}^{+} \mathrm{in}\mathrm{Kr}\right)=1.01\mathrm{}\pm 0.06$, $\mu \left({\mathrm{Xe}}^{+} \mathrm{in}\mathrm{He}\right)=18\mathrm{}\pm 1.1$, and $\mu \left({\mathrm{Xe}}^{+} \mathrm{in}\mathrm{Xe}\right)=0.55\mathrm{}\pm 0.03$ at ∼300°K. A study of the pressure dependence of the characteristic time constants of the electron density decay at fixed ratios of krypton to helium and xenon to helium concentrations yields the three-body conversion frequency of atomic krypton and xenon ions to their respective molecular ions.