Electron-impact energy dependence of the sodiumK-shell double-ionization cross section: A study of hypersatellite spectra

The K${\alpha }_2^h$ ($L^{\mathrm{-}n}$; n=0–5) hypersatellite spectrum of sodium has been detected for various electron-impact energies in the range of 15≤$E_0$≤200 keV. The measured energies of the transitions are in agreement with results of previous experiments at low electron energies and theoretical predictions. For the strongest hypersatellite, which corresponds to a $K^{\mathrm{-}2}$→$K^{\mathrm{-}1}$ $L^{\mathrm{-}1}$ transition, the natural width has been determined. The natural linewidth was found to be ${\Gamma }_K^h$=2.2±0.4 eV, which is much larger than predicted. Furthermore, the electron double-ionization cross section of the K shell has been obtained. Within the investigated energy region the cross section decreases by about a factor of 4 with increasing electron-impact energy. This energy dependence suggests the dominance of direct double ionization in comparison to the shake-off process.