Single-electron capture in intermediate energy He²⁺ + He collisions

A combined experimental and theoretical study on single-electron capture in 30 - 125 keV/u He²⁺ + He collisions was performed. Single-electron capture state-selective and angular-differential cross sections were measured using a cold target recoil ion momentum spectroscopy, for which the longitudinal momentum resolution has been improved up to 0.066 a.u.. More details of electron captured to higher excited-states and transfer-excitation processes were obtained from angular-differential cross sections. It was found that the experimental angular-differential cross sections were in good agreement with the two-active-electron semiclassical asymptotic-state close-coupling calculation for the considered impact energies. It is shown that single electron capture mainly occurs at small projectile scattering angles (< 0.4 mrad), where prominent oscillatory structures resulting from Fraunhofer-type diffraction can be seen. While a significant contribution from large projectile scattering angles (> 0.6 mrad) was observed in the transfer-excitation process at 30 keV/u. From a quasimolecular interaction picture, it is shown that both the small internuclear distance radial coupling and rotational coupling at the united-atom limit are responsible for the large angle scattering mechanism at 30 keV/u.