Electron impact double ionization of C- and O- ions

Absolute cross section measurements for double ionization of C- and O- ions by electron impact are reported. The animated crossed beams method has bee n employed in the energy range from ionization threshold to approximately 2 .5 keV. Present results are found to be smaller than the experimental resul t of Steidl et al (1995 Proc. 19th Int. Conf. Physics of Electronic and Ato mic 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 discr epancy. The study of double ionization processes along the fluorine isoelec tronic sequence (for O-, Ne+ and Ar9+ ions) shows that the results for mult iply 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 h igh energy. The autoionizing 1P state belonging to the (1s(2)2s2p(5)) confi guration of atomic oxygen is clearly seen to contribute to the O+ signal ab ove 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-.