Electron-impact ionization ofO5+: Improved measurements

New measurements of total cross sections for single ionization of ${\mathrm{O}}^{5+}$ by electron impact are reported in the energy range 120–580 eV. The improved experimental accuracy reveals the structure of the inner-shell excitation-autoionization threshold. Comparison with distorted-wave calculations for direct ionization shows good overall agreement, but up to 9% difference in the shape for direct ionization in the energy region below the onset of 1s-nl excitation-autoionization. An effort was made to measure structure in the cross section due to resonant-excitation–auto-double-ionization which was suggested by earlier measurements. No clear evidence of these processes is observed, and their contribution to the total single-ionization cross section is determined to be less than ${10}^{\mathrm{-}20}$ ${\mathrm{cm}}^2$.