The effect of resonances on the forbidden line of He-like ions

Effective collision strengths are presented for the forbidden line 1s 2 1 S 0 -1s2s 3 S 1 in He-like ions taking account of the effect of resonances and their radiative decay. Results are obtained using the quantum defect theory developed by Gailitis (1963) and Seaton (1969) and are applied to the ions O VII, Mg XI, Ca XIX and Fe XXV. Excitation rates are given in LSJ coupling for a temperature range where these ions are abundant and they are compared, where possible, with previous results. The results indicate that the contribution of the resonances to the direct collision strength is lower than 20% for the temperature range of interest.