Effective collision strengths for electron-impact excitation of nitrogen-like MG VI

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 $n=2$ states with configurations 2s$²$2p$³$, 2s2p$⁴$ and 2p$⁵$, and the four $n=3$ states with configuration 2s$²$2p$²$3s. 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{K} = 5.0 to log T{K} = 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. Table 5 is only available in electronic form at CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html