Ionization of carbon, nitrogen, and oxygen by electron impact

Total ionization cross sections of neutral carbon, nitrogen, and oxygen atoms by electron impact are presented. In our theory we have included the possibilities that ~a! some target atoms used in an experiment were in metastable states close to the ground state, ~b! excitation-autoionization of 2s2p exited states may be substantial, and ~c! ions produced in experiments may be in excited, low-lying metastable states. The binaryencounter Bethe ~BEB! model of Kim and Rudd @Phys. Rev. A 50, 3954 ~1994!# is used to calculate the cross sections for direct ionization. Plane-wave Born cross sections scaled by the method developed by Kim @Phys. Rev. 64, 032713 ~2001!# are used to determine the contributions from excitation-autoionization. A sum of the BEB cross sections for direct ionization weighted according to the statistical weights of the final ion states is used to modify the direct ionization cross sections. The combination of the BEB model and the scaled Born cross sections is in excellent agreement with available experimental data. The present method can easily be extended to heavier open-shell atoms.