Ionisation from the 4s, 4p, 4d and 4f sublevels of highly charged ions

Coulomb-Born-exchange calculations of the scaled or reduced cross section for ionisation from the 4s, 4p, 4d and 4f sublevels of hydrogenic ions with Z= infinity are made for several impact electron energies. The results are fitted to a simple form which is readily integrated over a Maxwellian distribution function. By following the prescriptions given in the authors earlier work (1978) the results can be easily used to obtain approximate cross sections for direct ionisation from the n=4 sublevels of any ion with finite Z for which Z/N>or approximately=2, where N is the number of bound electrons per ion.