| 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
3s
3p
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.