Ionisation from the 3s sub-level of highly charged ions

Scaled electron-impact cross sections are calculated for ionisation from the 3s sub-level of hydrogenic ions with Z= infinity using the Born exchange, or equivalently the Coulomb-Born-Oppenheimer approximation, which is exact, apart from relativistic corrections, in this limit. The results are fitted to an analytic expression which goes into the correct Bethe approximation result at high energies and which is readily integrated over a Maxwellian electron velocity distribution to obtain collision rates. From these results the approximate cross section and collision rate for ionisation from the 3s sub-level of any highly charged ion with Z/N>or approximately=2 can be readily obtained using a prescription given in earlier work. A more approximate procedure is also suggested for obtaining results for ionisation from the 3p sub-level. Numerical results for various ionisation processes in Fe 14+ and Fe 15+ obtained by the present procedures are compared with the results obtained by other methods.