Electron ionization of some low-Zions in the plane-wave Born approximation

Explicit plane-wave Born-approximation (PWBA) calculations are presented for electron ionization of ions of carbon, nitrogen, oxygen, and fluorine. Calculations of innershell excitation followed by autoionization are included. For ions of carbon, nitrogen, and oxygen, agreement with Coulomb-Born calculations is better than 15% except near threshold; agreement with experiment is better than 25% except near threshold, and except for the Be-like ions ${\mathrm{N}}^{3+}$ and ${\mathrm{O}}^{4+}$. For fluorine ions, explicit PWBA calculations are presented. These cross sections are as much as 40% higher than results obtained via a scaling procedure. A reexamination of the scaling mechanics indicates an inherent 15% error at high incident electron energy. An attempt is made to correct the error near the cross-section maximum by using an integral over the optical limit of the generalized oscillator strength and its momentum-transfer derivative. The correction is only partially successful because of significant changes in the shape of the generalized oscillator strength with increasing degree of ionization, near squared momentum transfer equal to ionization energy, and secondary electron energy much less than ionization energy.