ATOMIC DATA FROM THE IRON PROJECT .10. EFFECTIVE COLLISION STRENGTHS FOR INFRARED TRANSITIONS IN SILICON-LIKE AND SULFUR-LIKE IONS

Effective collision strengths for the electron impact excitation of transitions within the 3s(2)3p(q) ground configuration of ions in the silicon (q = 2) and sulphur (q = 4) isoelectronic sequences are presented. We consider here astrophysically abundant ions that give rise to infrared transitions within their P-3(J) ground term, namely, S III, Cl IV, Ar V, K VI, Ca VII, Ar III, K IV and Ca V. The configuration-interaction target approximations developed for the R-matrix calculations include single and double excitations within the n = 3 complex. An attempt is made to include a sufficient number of n = 3 target states in the close-coupling expansion so as to account for the prominent resonance effects that dominate the energy regions of interest, but development is handicapped since the energies of many of these states have not been measured. The contributions from resonances converging to n = 4 target states, which begin to appear in the energy region of interest for the lower members of the sequence, are studied in the case of S III. Relativistic effects in the target, which become important for high Z, are examined in Ca V. Both of these contributions to the excitation cross sections are found to be relatively small (< 20%) and therefore are neglected in the other systems under consideration. The good agreement found with previous calculations for S III and Ar III suggests that the present collisional data for the weakly ionised members of the sequence may be accurate to within 20%, but we expect this rating to deteriorate for higher Z due to the neglect of extra contributing thresholds.