Measurement of Cesium Excitation Cross Section near Threshold by a Swarm Technique

Electron-drift velocities have been measured in cesium-argon mixtures for ratios of electric field to total density between 3×${10}^{-19\mathrm{}}$ and 5×${10}^{-18\mathrm{}}$ V ${\mathrm{cm}}^2$ and cesium-to-argon concentration ratios between ${10}^{-8\mathrm{}}$ and ${10}^{-5\mathrm{}}$. The drift velocity was obtained from measurements of the electron transit time using an ac technique which is a modification of a technique developed originally by Rutherford for ion-drift-velocity measurements. Numerical solutions of the Boltzmann transport equation were used to determine an excitation cross section which is consistent with the experimental drift-velocity data. For a single excitation threshold at 1.386 eV, the best slope for a linear cross section is 7.1×${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$/eV. If excitation to the $6P_{\frac{1}{2}}$ and $6P_{\frac{3}{2}}$ states is considered separately, with thresholds at 1.386 and 1.454 eV, respectively, the linear cross sections obtained have a slope of 2.5×${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$/eV for $6P_{\frac{1}{2}}$ excitation, and 5.0×${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$/eV for $6P_{\frac{3}{2}}$ excitation. The range of validity of these values is from threshold up to about 1.8 eV. Other shapes for the cross section are investigated. The experimental cross section is in reasonable agreement with extrapolations of theoretical cross sections.