Oscillator strengths and effective collision strengths for electron excitation of Mg VI^⋆

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

Received: 6 June 2012
Accepted: 2 October 2012

Abstract

Aims. Electron impact excitation collision strengths and oscillator strengths for the astrophysically important lines in Mg VI are reported. Thermally averaged collision strengths are presented as a function of electron temperature for application to solar and other astrophysical plasmas.

Methods. The collision strengths were calculated in a close-coupling approximation using the B-spline Breit-Pauli R-matrix method. The multiconfiguration Hartree-Fock method with term-dependent, non-orthogonal orbitals was employed for an accurate representation of the target wave functions. The close-coupling expansion includes 74 bound levels of Mg VI covering the n = 2 and n = 3 terms. The present calculations led to a total of 2701 forbidden, intercombination, and allowed transitions between fine-structure levels. The effective collision strengths were obtained by averaging the electron collision strengths over a Maxwellian distribution of velocities.

Results. The accuracy of present oscillator strengths is evaluated by the agreement between the length and velocity formulations combined with the agreement between the calculated and measured excitation energies. The calculated excitation energies are in excellent agreement with experiments and other extensive configuration-interaction calculations. The oscillator strengths for all E1 transitions are listed. The effective collision strengths are tabulated for all 2701 transitions among the 74 fine-structure levels at 11 electron temperatures in the range from 10 000 to 200 000 K. The present results are compared with other close-coupling and distorted-wave calculations. Overall good agrement is generally found with the calculations by Ramsbottom & Bell and by Landi & Bhatia for many transitions, but significant differences are also noted for some transitions.