ELECTRON-IMPACT DOUBLE-IONIZATION OF SINGLY CHARGED IONS C+, N+, O+, F+ AND NE+

The first absolute cross section measurements for double ionization of C+, N+, O+ and Ne+ ions by electron impact are reported. The animated crossed beams method has been employed in the energy range from below ionization thresholds to approximately 2500 eV. The classical binary encounter approximation overestimates measured cross sections by almost two orders of magnitude. Along the sequence, the cross section maximum does not follow classical scaling laws. A simple scaling law based on an electron pair ejection model is proposed for the prediction of direct double ionization. Inner-shell ionization followed by autoionization is seen to play a dominant role for C+ only.