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

Abstract

Eective collision strengths for electron- impact excitation of the N-sequence ion Mg vi are eval- uated in the close-coupling approximation using the mul- tichannel R-matrix method. Twelve LS target eigenstates are included in the expansion of the total wavefunction, consisting of the eight n = 2 states with congurations 2s 2 2p 3 ,2 s2p 4 and 2p 5 ,a nd the four n= 3 states with con- guration 2s 2 2p 2 3s. These 12 ionic states correspond to 23 ne-structure levels, leading to a total of 253 independent transitions. The eective collision strengths for all possible forbidden, semi-forbidden and allowed transitions are ob- tained by averaging the electron collision strengths, for a wide range of incident electron energies, over a Maxwellian distribution of velocities. Results are tabulated 1 for elec- tron temperatures, T , in the range log T (K) = 5.0 to logT (K) = 6.1, which are the temperatures of interest for many astrophysical applications. We conclude that the ef- fective collision strengths, for many of the forbidden lines considered, are signicantly enhanced by autoionizing res- onances converging to the target state thresholds in the collisional cross sections. The present calculations are the rst to include such resonance phenomena.