Production of Metastable Ions in Cesium-Atom-Electron Collisions

The structure in the total electron-impact ionization cross section in cesium can be partially accounted for by the mechanisms of autoionization and excitation ionization of $5p$ electrons. For electron energies above 17 eV a large fraction of the ions are metastable and can de detected by Auger emission from a metal surface. The experiment was performed in a cesium-atom—electron crossed-beam apparatus, and the metastable ions were counted with a channel electron multiplier. The metastable-ion count rate was a factor of 100 higher than that due to photons from atomic and ionic transitions. We have measured the over-all excitation function for a number of unresolved ${\mathrm{Cs}}^{+}$ metastable levels, the lowest being $5d\left\{\right[3\frac{1}{2}],3\}$ at 17.02 eV and $5d\left\{\right[\frac{1}{2}],1\}$ at 17.06 eV.