Single ionisation of multiply charged xenon ions by electron impact

Abstract

Single-ionisation cross sections sigma _q,q+1 for electrons incident on Xe^q+ ions (q=1, 2, 3, 4) have been measured for electron energies up to 700 eV by employing a crossed beams technique. The results for Xe³⁺ ions agree with previous experimental data of Gregory et al. (1983). The obtained cross sections increasingly exceed distorted wave with exchange (DWX) calculations of Younger (1980) for direct ionisation by factors of up to about five when the charge state of the parent ions is increased from q=1 to q=4. The cross sections sigma _3,4 and sigma _4,5 show an unexpected enhancement at electron energies around 100 eV. These features are attributed to indirect ionisation processes, which probably also cause another unusual result: for the first time the authors found a case where ionisation of an atomic ion becomes easier when its charge state is increased, namely, for electron energies above 250 eV sigma _4,5 is greater than sigma _3,4.