4 The carbon isoelectronic sequence

4.1 AlVIII and NiXXIII

The adopted Al VIII and Ni XXIII models include the three n=2 configurations ([2s 2][2p 2], [2s2p 3] and [2p 4]) corresponding to 20 fine-structure energy levels. Experimental energy levels are taken from the NIST database (Martin et al. 1995), with the exception of the [2s2p 3] [3 P ] levels coming from Edlen (1985) in the case of Al VIII. Theoretical energy levels come from Zhang & Sampson (1996).

Radiative transition probabilities come from Zhang & Sampson (1996) (electric dipole) and Galavis et al. (1997) for the ground levels. Zhang & Sampson (1996) relativistic Distorted Wave collision strengths have been adopted; they provide collision strengths for all possible transitions calculated for six values of the incident electron energy.

4.2 SiIX

The R-matrix calculations of Aggarwal (1983) have been used to provide collisional data for the ground transitions of Si IX. For all other transitions, the Distorted Wave collision strengths of Bhatia & Doschek (1993, 1995) have been retained. The inclusion of resonances for the ground transitions have significant effects in the

determination of densities from the Si ix] density diagnostics.