Abstract
Absolute cross sections sigma q,q+1 for electron impact single ionisation of Krq+ ions (q=1, 2, 3) have been measured for electron energies up to 700 eV by employing a dynamic crossed-beams technique. The experimental data significantly exceed the Lotz prediction for direct ionisation of Kr2+ and Kr3+ ions, indicating the importance of indirect processes for multiply charged Krq+ ions. The observed ionisation thresholds of cross sections from the present measurements as well as of data from Danjo et al. (1984) and Gregory et al. (1983) indicate the presence of metastable ions in the parent ion beams. The influence of metastable states on the cross sections is discussed on the basis of Lotz calculations (1968) for direct ionisation from different excited states and differences observed in the experiments are rationalised.