The collision strength ,
for the ground term fine structure transition, is computed for fluorine like
ions from Mg IV to Ni XX at a fine enough energy mesh for one to be
able to integrate accurately the product of times the
Maxwell velocity distribution function. The energy range covered suffices to
obtain *effective collision strengths* up to and above the temperatures of
maximum ionic abundance as compiled by
Arnaud & Rothenflug (1985).
The excitation rate coefficient in is

where is the statistical weight of the ground level and
is Boltzmann's constant with the value
.
Numerical values of the fine structure transition energy
are given in Col. (c) of Table 4 for all elements in the isoelectronic
sequence as far as nickel. We obtained these values using
Edlén's (1969)
formulas which accurately fit the observed intervals for and should
provide reliable estimates for ions with .

A comparison with the work by Mohan and co-workers, whose level of approximation is similar to ours, has shown the existence of some major disagreements. But we have been able to pinpoint the reasons for these and show that our results are more reliable than theirs.

*Acknowledgements*

This work was supported by the PPARC grant GR/H93576, and the EC network contract ERB CHRX CT920013. All of the figures were drawn using the graphics package called TVB ("Tracer vite et bien'') which the author, Georges Gonczi of Nice Observatory, kindly made available. We wish to thank Alan Burgess and Alberto Binelli of Cambridge University and Werner Eissner of Bochum University for their constructive comments on an earlier version of the paper.

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