It is shown that two parallel ion beams react at a rate that is independent of their density profiles when made to oscillate against each other in a two-dimensional scanning motion. An experimental set-up that makes use of this principle is described. Absolute cross sections obtained in this way are in good agreement with those obtained with the beams merging in the usual (static) mode. Cross sections for single-charge transfer between and in the energy range 5 - 4000 eV are presented and compared to other existing data.

The cross section for one-electron transfer H − + He + → H(1s) + He * (1s, nlm) is calculated in this paper. The collisional system is treated as a three-electron one and matrix elements for the formation of excited helium He * (1s, nlm) in both singlet and triplet states (1S, 2 1,3 S, P and 3 1,3 S, P, D) are obtained. Close-coupling calculations were done for the 27 1s, nlm| (1S, 2 1,3 S, P and 3 1,3 S, P, D) final states covering the range 1-10 × 10 7 cm s −1 of the relative collision velocity. An approximation is used for the effective potential of H − and Coulomb Green's functions are used to describe the weakly bound electron of H − . A satisfactory agreement is obtained with the experimental cross section.

Measurements were carried out using merged beams and chopping the negative beam to evaluate the background. The measurements cover the barycentric energy range from 0.07 to 10 eV. The four cross sections were, within experimental error, equal both in magnitude and energy dependence.

Absolute cross sections of the associative ionization in the collision of He + with H − or D − have been measured in a merged beam experiment over a relative velocity interval ranging from 0.4 to 5 × 10 6 cm s −1 . At a given relative velocity, the two cross sections significantly differ at low velocities where the cross section for HeD + formation exceeds that of HeH + by as much as 35%. The difference, however, decreases with the velocity and falls in the experimental error above 2 × 10 6 cm s −1 . In both cases the energy dependence becomes compatible at low energy with an E −1 law.

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