Electron-impact ionization of the oxygen atom

The cross sections for electron-impact ionization of oxygen atoms in the (2p⁴{)}³$P ground state to form ${O}⁺$ ions in the (2${p}³)⁴S, (2p³{)}²$D, and (2${p}³)²P states have been calculated by the Born-Ochkur approximation for incident-electron energies from the threshold to 500 eV and by the method of exchange distorted waves (with Ochkur's approximation) from the threshold to 200 eV. Comparisons are made with previous theoretical calculations and with experimental measurements. Our calculated cross sections agree well with the experimental values, the difference being about 15% at energies above 50 eV. The contributions from autoionization and from the inner-shell ionization (by removing the 2s electron) are also discussed. The cross-section calculation is extended to ionization of oxygen atoms in the [2p³{(}⁴$S)${n}₀l₀{]}³L₀ excited states by means of the Born-Ochkur approximation for n₀{l}₀$ up to (6,5) and the method of distorted waves for ${n}₀l₀=3s, 3p, and 3d.