The Dirac R-matrix theory is used to calculate the collision strengths for electron impact excitation of He-like ions (S 14+ , Ca 18+ and Fe 24+ ) using the Dirac atomic R-matrix code, which has been developed mainly by Norrington and Grant (Norrington P H and Grant I P 1987 J. Phys. B: At. Mol. Phys. 20 4869). The lowest 31 target levels are included in the calculation. Rate coefficients are obtained for transitions from the ground 1s 2 1 S 0 state to the fine-structure levels of all excited states of 1s2 ( = 0, 1) and 1s3 ( = 0, 1, 2) configurations. We compare our results for rate coefficients with the results of previous calculations, and indicate where significant differences arise because of our more accurate representation of resonance and relativistic effects.

Abstract. Electron excitation collision strengths for all the transitions among 3s, 3p, 3d, 4s, 4p, 4d, 4f states of the five Na-like ions Al III, Si IV, S VI, Ar VIII and Ca X are calculated in the R-matrix approximation. Eleven target terms are included in the expansion of the total wavefunction. These collision strengths are averaged over a Maxwellian velocity distribution to obtain effective collision strengths as a function of electron temperature. The resulting values are compared with the results of previous less elaborate calculations to find some disagreements.

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Collision strengths for electron impact excitation of S xv are calculated in a close-coupling approach. The nineteen lowest LS states are included in the calculation. Rate coefficients are obtained for transitions from the ground 1s 2 1 S 0 state to the fine-structure levels of all excited states of 1s2 ( = 0, 1) and 1s3 ( = 0, 1, 2) configurations. The resulting effective collision strengths are fitted to a polynomial function of log(T e /K), and rate coefficients are presented in the temperature range between 1 10 6 K and 1 10 8 K.

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