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 tr ansitions within the 3s(2)3p(q) ground configuration of ions in the si licon (q = 2) and sulphur (q = 4) isoelectronic sequences are presente d. We consider here astrophysically abundant ions that give rise to in frared 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-inter action target approximations developed for the R-matrix calculations i nclude single and double excitations within the n = 3 complex. An atte mpt is made to include a sufficient number of n = 3 target states in t he close-coupling expansion so as to account for the prominent resonan ce effects that dominate the energy regions of interest, but developme nt is handicapped since the energies of many of these states have not been measured. The contributions from resonances converging to n = 4 t arget states, which begin to appear in the energy region of interest f or 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 agre ement found with previous calculations for S III and Ar III suggests t hat 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 thre sholds.