Electron impact excitation cross sections of Mg II calculated in a four-state close-coupling approximation

The electron impact excitation cross section for the resonance transition of Mg II is calculated in a four-state close-coupling approximation. The interaction Hamiltonian contains a model potential that accounts for core polarisation effects. A detailed analysis of the resonance series converging to the 4s 2 S and 3d 2 D thresholds is made using quantum defect theory techniques. The total cross section is compared with experiment and other calculations, and rate coefficients are presented for electron temperatures of 5000-20000K. Cross sections are also calculated for the transitions 3s-3d, 3p-3d, 3s-4s, 3p-4s, 4s-3d and 3p 2 P 1 2/ 0 -3p 2 P 3 2/ 0 .