The double electron capture process by He'+ ions in collisions with helium is studied in the high velocity regime with the measurement of total cross sections at 1.5, 4 and 6 MeV beam energy and angular differential cross sections far 1.5 MeV. The experimental results compare well with theoretical calculations which include correlation effects in the initial and final He ground states for the derivation of the capture amplitude in a continuum distorted-wave approximation.Electron capture processes in fast ion-atom collisions are of great interest both from theoretical and experimental points of view. One of the interests in studies of this process arises from the difficulties in describing the rapid transfer of the electron from a target hound state to a projectile bound state at high velocities (see e.g. Shakeshaft and Spruch 1979). In the case of non-radiative capture this requires a considerably

We have measured electron-ion recombination rates for bare ions of D + , He 2+ , N 7+ , Ne 10+ and Si 14+ in a storage ring. For the multi-charged ions an unexpected energy dependence was found, showing a strong increase of the measured rates over the calculated radiative recombination rate for electron beam detuning energies below the electron beam transverse temperature. The measured enhanced rates increase approximately as Z 2.8 with the charge state Z. A comparison of these rates with theoretical predictions for collisional-radiative recombination in the cold magnetized electron plasma, in particular three-body recombination including radiative de-excitation of electrons in Rydberg levels, is made.

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We present measurements of the angular distribution of fast hydrogen atoms formed by electron capture of 2.8-and 5.0-MeV protons in atomic hydrogen. In the angular region of the Thomas peak (0.47 mrad) the experimental results obtained with this pure three-body collision system are in reasonable agreement with a strong-potential Born calculation and the impulse approximation, but not with other higher-order theories. PACS numbers: 34.70.+eAt present there is great interest in electron capture in ion-atom collisions at high projectile velocity. High velocity here means that the projectile velocity is much higher than the Bohr electron orbital velocities in the initial and final states. Because of the rapid transfer of the electron from the target bound state to the fast-moving projectile bound state, a large change of electron momentum and energy is required which must be transferred to a third particle, which can be, in the case of nonradiative capture, the target nucleus. This process is theoretically attractive, 1,2 since higher-order terms of the electronnucleus interaction become important in order to mediate this energy and momentum transfer. Formally, higherorder terms can be described by perturbative series expansions like the Born series. In the last few years different approaches to these series expansions have been developed, e.g., second-order Born (B2), 3 " 5 strongpotential Born (SPB), 6 " 9 impulse (IA), 10 " 12 continuum distorted-wave (CDW), 13 ' 14 and eikonal approximations (EA). 15 The higher-order terms in the capture amplitude can be interpreted as multiple scattering of the electron at the target and projectile potentials. In this view the secondorder term is a double scattering for the capture of the electron, where the electron is first scattered at the projectile and then at the target nuclear potential as described first classically by Thomas. 16 This leads to a peak at a forward angle 9*°* (m/M p )sin60° (m is the electron mass; M p is the projectile mass) in the angulardifferential capture cross section. The shape and absolute magnitude of the differential cross section may be determined not only by first-and second-order, but also by higher-order terms in the series expansions mentioned above. 12 However, in such treatments and at high collision velocities the description of the asymptotic states even to first order is a difficult task. 17 The ultimate experimental test of these theories can only be performed with a pure three-body collision system. We have therefore studied the three-body collision system /?-H by measuring the angular-differential electron-capture cross sections at 2.8 and 5.0 MeV.Among the different experimental approaches to test higher-order contributions to the electron-capture amplitude were studies of the electron cusp from capture into projectile continuum states. In these experiments, some signature of higher-order terms could already be identified. 18 ' 19 In total cross sections of electron capture the higher-order terms might dominate the first-order term ...

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