Electron impact ionisation of Li-like and Be-like ions

Electron impact ionisation cross sections for positive ions in the Li and Be isoelectronic sequences are calculated in the Coulomb-Born exchange and distorted-wave exchange approximations - using close-coupling wavefunctions to represent the initial and final states of the target system. The corresponding ionisation rate coefficients are also computed as a function of temperature. The results obtained are compared with experiments, with the infinite-Z method of Golden and Sampson (1979) and with the Lotz semiempirical formula. For C, N, O and Ne ions the mutual agreement is better than 25% and in some cases is of the order of a few per cent. For Fe 23+ and Fe 22+ the Lotz formula gives rates up to 40% higher than the Coulomb-Borm at high temperatures (>10 8 K), indicating that the formula probably gives an inaccurate estimate of the high-energy behaviour of the cross section for these ions.