Electron impact excitation collision strength for transitions in C II^*

Department of Physics, Clark Atlanta University, Atlanta, GA 30314, USA e-mail: stayal@cau.edu

Received: 25 April 2008
Accepted: 20 May 2008

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, 2s^23l (), 2p³, 2s^24l (–3), 2s2p3s and () 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°–2s2pD, ²S and intercombination ²P°–2s2p^2 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.