Impact parameter dependence of electron capture in slow O5+ + Hecollisions

Abstract

Angular distributions of recoiling He2+ andHe+ions following collisions of slow O5+ions and He atoms were measured, for projectile energies in the range100–2500 eV. The recoil ions were detected at angles from 20° to130°.Two groups of peaks are clearly visible, corresponding tosingle and double electron capture. Highly energetic He2+ andHe+ions were observed at forward angles. It has already been shown in previous works on the C5+ and B5+ + He collisions thatrecoiling He+targets mainly originate from a single electron capture on the n = 2or 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 calculationsbased on classical kinematics equations. From this model, charge exchange probabilitiesP (b),where bis the impact parameter, were deduced for the explored projectile energies. Toexplain the large kinetic energies for recoil targets, we had to invoke electroncapture processes under small impact parameter conditions (b < 1 au).At 2500 eV, while the capture events occurring at large impact parameters are notdetected in the present measurements, electron capture in collisions at smallimpact parameters is evidenced. At the lowest projectile energy (100 eV) wehave access to the whole range of impact parameters. The contribution ofsmall impact parameters is shown to be dominant at this energy.