For the first time, charge-changing reactions in collisions of two negative ions were investigated. Absolute cross sections for mutual ionization were determined for H− colliding with H−, using crossed beams and coincident detection of the reaction products. The centre-of-mass energy range covered in the experiment extended from 1.5 keV to 90 keV for the reaction channel H− + H− to H0 + H0 + 2e− and from 4 keV to 40 keV for the channel H− + H− to H0 + H+ + 3e−. The measured cross sections are compared with results of CTMC calculations obtained with different model potentials for the interaction between the outer electron and the H0 core

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Using the crossed-beams technique, we have measured absolute cross sections for the total production of in collisions between and ions by means of a beam-pulsing method for the detection for centre-of-mass energies between 1.4 and 39.8 keV. From the measured data cross sections for the electron detachment process were calculated using the total cross sections for mutual neutralization measured previously. The electron detachment results are in very good agreement with recent theoretical calculations.

An inte-tingbeam technique has been used to measure total cross Sections for resonant charge transfer in 3He2t-4Het collisions at bwcenvic energies between 4 and 200 keV. Our measurements. obtained using coincident detection of the reaction products, are in good apement with theoretical predictions. Funhermore, h e y confirm a scaling law by Reinhold and Falcdn for symmetric reSonant charge transfer in one-elecuon systems at intermediate energies.The interest in charge-changing collisions between positive ions arises primarily from applications in fusion plasmas. These collisions are important in ion confinement, losses and alpha-particle heating of fusion plasmas.This paper describes the resonant charge-transfer reaction 'HeZ+ + 4He+ + 'Het + 4He2+(1) in the centre-of-mass energy range 4-200 keV.This reaction represents a unique collision system due to the following reasons. First, it is a one-electron system which is comparatively easy to describe and therefore an ideal testing ground for theory. Second, it is a resonant and a charge-symmetric system. Large cross sections would be expected at moderate collision energies. Third, it is the simplest ion-ion collision system with Coulomb repulsion bolh in the incoming as well as in the outgoing channel leading to remarkable angular scattering of the reaction products.Jognaux et a f (1978) and Peart and Dolder (1979) have performed measurements for this charge-transfer reaction at CM energies of 0.01-1.7 keV and 0.1-20 keV, respectively. Both measuremenls made use of the 3He isotope for the doubly-charged ions, because H i ions have a virtually identical ratio of charge to mass to that of the 4He2t ion. Calculations spanning the energy range 0.01-17 keV have been performed by Dickinson and Hardie (1979). Forster et al (1988) and Bardsley etal (1989).In the present experiments, the experimental set-up and the measuring procedure are the same as described in detail in a previous paper (Rinn el af 1985). Briefly, two momentumanalysed ion beams of adjustable energies (slow beam: IO keV, fast beam: 20-421 keV) are made to intersect at an angle rP = 45" in an ultra-high vacuum of a few lo-'' mbar. The collision products, charge-analysed downstream, from the interaction region, are counted in coincidence by single-particle detectors. In the fast beam, 'Het ions are separated from * Work supported by BMFT under contract no 06 Gi 333.

A crossed-beams experiment designed to study charge-changing collisions between singly charged ions has been modified to extend its applicability to collisions between multiply charged ions in a broader range of collision energies. Computer simulations of the beam transport system and the new electrostatic analyzing units are presented. The ion-optical predictions are tested experimentally. A data acquisition system that allows measurements of angular differential cross sections in ion-ion collisions is described.