Impact parameter dependence of electron capture in slow O⁵⁺ + He collisions

Angular distributions of recoiling He²⁺ and He⁺ ions following collisions of slow O⁵⁺ ions and He atoms were measured, for projectile energies in the range 100–2500 eV. The recoil ions were detected at angles from 20° to 130°. Two groups of peaks are clearly visible, corresponding to single and double electron capture. Highly energetic He²⁺ and He⁺ ions were observed at forward angles. It has already been shown in previous works on the C⁵⁺ and B⁵⁺ + He collisions that recoiling He⁺ targets mainly originate from a single electron capture on the n = 2 or 3 orbitals of the projectile, whereas He²⁺ ions are due to a double electron capture on 2ℓnℓ'(n ≥ 2) configurations. The population of such configurations is confirmed by our model calculations based on classical kinematics equations. From this model, charge exchange probabilities P (b), where b is the impact parameter, were deduced for the explored projectile energies. To explain the large kinetic energies for recoil targets, we had to invoke electron capture processes under small impact parameter conditions (b < 1 au). At 2500 eV, while the capture events occurring at large impact parameters are not detected in the present measurements, electron capture in collisions at small impact parameters is evidenced. At the lowest projectile energy (100 eV) we have access to the whole range of impact parameters. The contribution of small impact parameters is shown to be dominant at this energy.