A measurement of the cross section for electron impact ionisation of Ne + , Ar + , Kr + and Xe +

The crossed electron-ion beams technique has been used to measure the absolute cross sections for the single ionisation of Ne + , Ar + , Kr + and Xe + ions at electron energies ranging from the ionisation threshold to 2000 eV. The total error in the cross sections at a 90% confidence level is estimated to be +or-5% or less at energies more than a few eV above threshold. In contrast to some previous measurements, the metastable contents of the ion beams are negligible except in the case of Xe + . The cross section curves for Ar + , Kr + and Xe + show significant contributions from excitation-autoionisation and possibly direct ionisation of inner-shell electrons. The present results for Ne + and Ar + are in excellent agreement with those previously measured in the laboratory. Good agreement is also obtained with the more recent data from the Giessen group for Ne + , Ar + and Xe + . Comparisons with theoretical calculations show that the scaled Born approximation of McGuire tends to underestimate the cross sections at all energies, but especially at low energies. The semi-empirical formula of Lotz also underestimates the cross sections at low energies. Comparisons are made with other available experimental and theoretical calculations.