Electron impact excitation collision strength for transitions in C II

Abstract

Aims. Oscillator strengths and electron impact excitation collision strengths for C II lines among the fine-structure levels are calculated. Thermally averaged collision strengths are presented as a function of electron temperature for application to astrophysical plasmas. Methods. An accurate description of the target wave functions has been obtained in the multiconfiguration Hartree-Fock approach with flexible non-orthogonal orbitals. The 42 fine-structure levels of the 2s22p, 2s2p2, 2s23l (l = 0 − 2), 2p3, 2s24l (l = 0–3), 2s2p3s and 2s25l (l = 0− 2) configurations have been included in the scattering calculation. A second calculation with lowest 35 levels in the close-coupling expansion has also been carried out to check channel coupling effects on collision strengths. The continuum functions have been represented by the use of B-spline basis. The B-spline R-matrix method in the framework of Breit-Pauli approximation has been used to investigate the electron impact excitation of forbidden and allowed transitions in C II. Results. The present cross sections for the resonance 2P◦–2s2p2 2D, 2S and intercombination 2P◦–2s2p2 4P transitions are in very good agreement with the electron energy-loss merged-beams experiment. Oscillator strengths and transition probabilities for C II lines normally compare very well with previous calculation. The effective collision strengths are obtained by integrating total resonant and non-resonant collision strengths over a Maxwellian distribution of electron energies and these are presented over a wide temperature range suitable for modeling of astrophysical plasmas. Our effective collision strengths show significant differences with those from previous calculations for transitions involving higher excitation levels.