1 Introduction

Zhang et al. (1990) (hereafter referred to as ZSF) give extensive tables of collision strengths ${\sl \Omega}$ for transitions in Li-like ions with atomic numbers Z₀ ranging from 8 to 92. They obtained their data for $Z_0 = {\rm 8, 14, 22, 30, 42, 56, 74, 92}$ by making detailed calculations with a relativistic distorted wave approximation. They then used an eight parameter analytic fit in Z₀ to complete their tabulation for the remaining ions in the sequence between O⁺⁵ and U⁺⁸⁹.

In order to simplify the storage of these data we examine the possibility of fitting them, both as functions of energy and of ion charge number Z. We then carry out thermal averaging and fit the resulting effective collision strengths ${\sl \Upsilon}$ as functions of temperature T and Z. The basis for carrying out each of these processes was set out in detail by Burgess & Tully (1992) (hereafter referred to as B&T) and we use their notation. B&T also described the interactive program OMEUPS and used it to obtain such isoelectronic fits for the 2s-2p transition in all the Li-like ions of charge 1 to 23, and for the 2¹S-2¹P transition in all the Be-like ions of charge 2 to 10.

Our present purpose is to extend the treatment in three ways:

First, we fit the ZSF data for the non-optically allowed transitions 2s-3s, 2s-4s, 2s-5s, 2s-3d, 2s-4d and 2s-5d. To help achieve this, we calculate the Born high energy limit points following the procedure described by Burgess et al. (1997).

Secondly, we incorporate some of the relativistic corrections required for larger values of Z than those treated in B&T.

Finally, we illustrate the use of types of fit other than the 5-point cubic spline adopted as standard in the previous work and in current plasma modelling computer programs.

We use a recent version of the interactive program, renamed OmeUpZ, designed to handle these extensions. Details will be given elsewhere (Burgess 2001).