Breit-Pauli oscillator strengths and electron excitation collision strengths for Si VIII^⋆

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

Received: 8 February 2012
Accepted: 19 March 2012

Abstract

Aims. Oscillator strengths and electron impact excitation collision strengths for transitions between the 68 fine-structure levels of the 2s²2p³, 2s2p⁴, 2p⁵, 2s²2p²3s, 2s²2p²3p, 2s²2p²3d and 2s2p³3s configurations in Si VIII are calculated. Thermally averaged collision strengths are presented as a function of electron temperature for application to solar and other astrophysical plasmas.

Methods. The collision strengths have been calculated using the B-splineBreit-Pauli R-matrixmethod for allowed and forbidden transitions in Si VIII. The relativistic effects have been incorporated through mass, Darwin and spin-orbit one-body operators in the Breit-Pauli Hamiltonian in the scattering calculation, while in the calculation of oscillator strengths the one-body and two-body relativistic operators are included. Flexible non-orthogonal sets of spectroscopic and correlation radial functions are used to obtain accurate description of Si VIII levels and to represent the scattering functions. The 68 fine-structure levels of the 2s²2p³, 2s2p⁴, 2p⁵, 2s²2p²3s, 2s²2p²3p, 2s²2p²3d and 2s2p³3s configurations have been considered in both the radiative and scattering calculations. The present scattering calculations are more extensive than previous ones, leading to a total 2278 transitions between fine-structure levels.

Results. The calculated excitation energies are in excellent agreement with experiment and represent an improvement over the previous calculations. The present collision strengths show reasonable agreement with the previously available R-matrix and distorted-wave calculations. The oscillator strengths for E1 transitions normally compare very well with previous calculations. 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 from 10⁴ to 4.0 × 10⁶ K.