Electron impact double ionization of C- and O- ions

Belenger, C. Belic, DS. Yu, DJ. Defrance, Pierre [UCL]

Absolute cross section measurements for double ionization of C- and O- ions by electron impact are reported. The animated crossed beams method has been employed in the energy range from ionization threshold to approximately 2.5 keV. Present results are found to be smaller than the experimental result of Steidl et al (1995 Proc. 19th Int. Conf. Physics of Electronic and Atomic Collisions (Whistler) ed J B A Mitchell et al p 564). The role of slow positive ions trapped in the electron beam could explain the observed discrepancy. The study of double ionization processes along the fluorine isoelectronic sequence (for O-, Ne+ and Ar9+ ions) shows that the results for multiply charged ions are strongly lower than predicted by classical scaling. A systematic threshold energy shift is observed in the sequence and, for O-, the position of the cross section maximum is found to be at an unusually high energy. The autoionizing 1P state belonging to the (1s(2)2s2p(5)) configuration of atomic oxygen is clearly seen to contribute to the O+ signal above 23.7 eV. Other autoionizing states are also seen to play a role around the K-shell ionization threshold. The Bethe-plot of present data shows that the high energy behaviour of cross sections is dominated by the E-1, term for C-, while it is dominated by the logarithmic term for O-.