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 $n=3\mathrm{}$ 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 $2s2p^{m+1\mathrm{}}$ and $2p^{m+2\mathrm{}}$. 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 ${\mathrm{C}}^{-}\left({}_{}{}^4{}_{}{}^{}S_{}^o{}_{}{}^{}\right)$ bound and ${\mathrm{O}}^{-}\left({}_{}{}^2{}_{}{}^{}P_{}^o{}_{}{}^{}\right)$, the CI calculations give qualitative predictions of the true location of other $2s^{2}2p^{m+1\mathrm{}}$ states: ${\mathrm{C}}^{-}\left({}_{}{}^2{}_{}{}^{}D_{}^o{}_{}{}^{}\right)$ bound; ${\mathrm{C}}^{-}\left({}_{}{}^2{}_{}{}^{}P_{}^o{}_{}{}^{}\right)$ resonance near 1 eV; ${\mathrm{N}}^{-}\left({}_{}{}^3{}_{}{}^{}P\right)$ very near threshold, either weakly bound or a very narrow resonance; no ${\mathrm{O}}^{-}$ resonance. A method for precise location of these negative-ion states is proposed.