Matrix variational calculations of electron scattering by C, N, and O atoms

Total elastic cross sections for electron scattering by ground-state C, N, and O atoms have been computed by the matrix-variational method for energies up to the first n=3 excitation. Two levels of approximation are considered: single configuration (SC) and near-degenerate configuration interaction (CI). Both approximations neglect the principal effects of target-atom electric-dipole polarizability. The SC calculations are the variational equivalent of earlier close-coupling calculations including all states of the ground configuration 2s^2 2p^m. The CI calculations augment this with configurations 2s 2p^(m+1) and 2p^(m+2). The short-range correlations introduced by the CI calculations are found to have dramatic effects on resonance structures found at low energies in SC calculations. In addition to the well-established negative-ion bound states C^-(^4S^o) bound and O^-(^2P^o), the CI calculations give qualitative predictions of the true location of other 2s^2 2p^(m+1) states: C^-(^2D^o) bound; C^-(^2P^o) resonance near 1 eV; N^-(^3P) very near threshold, either weakly bound or a very narrow resonance; no O^- resonance. A method for precise location of these negative-ion states is proposed.