Effective collision strengths for electron-impact excitation of nitrogen-like Mg vi ^*

Department of Applied Mathematics and Theoretical Physics, The Queens University of Belfast, Belfast BT7 1NN, UK

Send offprint request to: C.A. Ramsbottom

Received: 13 January 1997
Accepted: 19 January 1997

Abstract

Effective collision strengths for electron-impact excitation of the N-sequence ion Mg vi are evaluated in the close-coupling approximation using the multichannel R-matrix method. Twelve LS target eigenstates are included in the expansion of the total wavefunction, consisting of the eight states with configurations 2s²2p³, 2s2p⁴ and 2p⁵, and the four states with configuration 2s²2p²3s. These 12 ionic states correspond to 23 fine-structure levels, leading to a total of 253 independent transitions. The effective collision strengths for all possible forbidden, semi-forbidden and allowed transitions are obtained by averaging the electron collision strengths, for a wide range of incident electron energies, over a Maxwellian distribution of velocities. Results are tabulated for electron temperatures, T, in the range log T() = 5.0 to () = 6.1, which are the temperatures of interest for many astrophysical applications. We conclude that the effective collision strengths, for many of the forbidden lines considered, are significantly enhanced by autoionizing resonances converging to the target state thresholds in the collisional cross sections. The present calculations are the first to include such resonance phenomena.