Using the cold target recoil ion momentum spectroscopy technique, we have measured state-selective projectile scattering angles for single and double electron captures in collisions of protons and He 1,2+ projectiles with a helium target for incident energies of 60-630 keV/u. We also report theoretical results obtained by means of four-body one-channel distorted-wave models ͑continuum distorted-wave Born final state, continuum distorted-wave Born initial state, and Born distorted wave͒ and find mixed agreement with the measured data.

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The differential and total cross sections for single charge exchange in p + He collisions have been calculated by means of the Born distorted wave (BDW-4B) approximation. The BDW-4B model is a four-body quantum-mechanical distorted-wave theory. The computations of total cross sections are carried out at impact energies ranging from 40 keV to 15 MeV. The differential cross sections are calculated at impact energies covering 50, 100, 150, 200, 293, 300 and 400 keV. The theoretical results obtained for the differential and total cross sections are compared with the available experimental data and good agreement is found.

Single-electron capture from heliumlike atomic systems by bare projectiles is investigated by means of the four-body boundary-corrected first Born approximation (CB1-4B). The effect of the dynamic electron correlation is explicitly taken into account through the complete perturbation potential. The quantum-mechanical post and prior transition amplitudes for single charge exchange encompassing symmetric and/or asymmetric collisions are derived in terms of two-dimensional real integrals in the case of the prior form and five-dimensional quadratures for the post form. An illustrative computation is performed for single-electron capture from helium by protons and α particles at intermediate and high impact energies. The role of dynamic correlations is examined as a function of increased projectile energy. The validity and utility of the proposed CB1-4B method is critically assessed in comparison with the existing experimental data for total cross sections, and excellent agreement is obtained.

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