Electron-impact excitation of carbon

Abstract

The $B$-spline $R$-matrix method is used to investigate the electron-impact excitation of neutral carbon over an energy range from threshold to $60\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. A multiconfiguration Hartree-Fock method with nonorthogonal orbitals is employed to generate an accurate representation of the target wavefunctions. The present close-coupling expansion includes the bound and autoionizing states of neutral carbon derived from the $1{s}^{2}2{s}^{2}2{p}^{2}$, $1{s}^{2}2{s}^{2}2p3\ensuremath{\ell}\phantom{\rule{0.3em}{0ex}}(\ensuremath{\ell}=0,1,2)$, $1{s}^{2}2{s}^{2}2p4s$, $1{s}^{2}2s2{p}^{3}$, and $1{s}^{2}2{p}^{4}$ configurations, plus eight pseudostates to fully account for the polarizability of the ground state of atomic carbon. Cross sections and effective collision strengths are presented for important transitions from the ground state $2{p}^{2}\phantom{\rule{0.3em}{0ex}}^{3}P$, and the metastable $2{p}^{2}\phantom{\rule{0.3em}{0ex}}^{1}D$ and $^{1}S$ states. Our predictions for the cross sections show significant discrepancies with those from previous calculations carried out with the standard $R$-matrix approach in a similar scattering model [Dunseath et al., J. Phys. B 30, 277 (1997)]. These discrepancies are mostly due to the different target descriptions, with the present one giving overall superior agreement with experiment for energy levels and oscillator strengths. The excitation cross sections exhibit prominent resonance structures in the low-energy region. The energy positions, widths, and classifications for the detected resonances are presented.