Abstract. The program developed by Burgess for spline fitting and interpolating collision strengths can also be used to do isoelectronic fits. Instead of treating Ω(E) or Υ(T) one inputs an arbitrary quantity A(Z) that is a function of nuclear charge number Z. In this way isoelectronic atomic data can be visualised and compacted in an easy manner which allows for accurate interpolation along a sequence. Four illustrative applications are provided.

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Collision strengths for optically forbidden transitions in positive ions excited by electron impact tend to finite limits as the energy of the colliding electron becomes infinitely great. This statement applies to theoretical data obtained by means of approximations which ignore relativistic effects. The high-energy limiting values are given by the Born approximation, which Bethe showed can be reduced to a closed expression requiring integration over momentum transfer and the radial distance of the atomic orbital. In the appendix we describe an accurate numerical procedure for evaluating this double integral and also show how to perform the Racah algebra which arises when mixed configurations are used to describe the target. We make applications to several isoelectronic sequences taking configuration interaction into account.

Collision strengths for electron induced transitions in the beryllium-like ion Fe +22 are calculated using the intermediate coupling frame transformation (ICFT) version of the R-matrix programs. Our target has 98 fine structure states 1s 2 nl n l S LJ corresponding to n = 2 and n = 2, 3, 4. The present calculation is for electron impact energies in the range 3.15 to 380 Ry. When T exceeds about ten million degrees one needs to take account of contributions to the thermally averaged collision strength Υ coming from electrons with energies in excess of 380 Ry. We discuss a way of allowing for these contributions. Values of Υ for all the transitions between the ground state and the excited states 1s 2 2l n l n l S L J , with n = 2, 3, 4 are tabulated as a function of log T. The temperature range 6.3 ≤ log T ≤ 8.1 is centred on log T = 7.1 which is approximately where Fe +22 has maximum abundance in ionization equilibrium. To the best of our knowledge these are the first R-matrix calculations for Fe +22 for excitations to the n = 3, 4 levels. Good agreement with previous distored-wave calculations is found. However, the resonance contributions have an important effect on the effective collision strengths and in turn on the level populations.