|On the accuracy of the effective collision strength for the electron impact excitation of the quadrupole transition in Ar III|
|M.E. Galavis, C. Mendoza and C.J. Zeippen|
|Astronomy & Astrophysics Supplement Series, 1998, Vol.133, pp.245-248|
Since Burgess et al. (1997) have recently questioned the accuracy of the effective collision strength calculated in the IRON Project for the electron impact excitation of the 3s3p D - S quadrupole transition in Ar III, an extended R-matrix calculation has been performed for this transition. The original 24-state target model was maintained, but the energy regime was increased to 100 Ryd. It is shown that in order to ensure convergence of the partial wave expansion at such energies, it is necessary to take into account partial collision strengths up to L=30 and to ``top-up'' with a geometric series procedure. By comparing effective collision strengths, it is found that the differences from the original calculation are not greater than 25% around the upper end of the common temperature range and that they are much smaller than 20% over most of it. This is consistent with the accuracy rating (20%) previously assigned to transitions in this low ionisation system. Also the present high-temperature limit agrees fairly well (15%) with the Coulomb-Born limit estimated by Burgess et al., thus confirming our previous accuracy rating. It appears that Burgess et al., in their data assessment, have overextended the low-energy behaviour of our reduced effective collision strength to obtain an extrapolated high-temperature limit that appeared to be in error by a factor of 2.