State-selective single- and double-electron capture in intermediate-energy in C4+ + He Collisions

The charge exchange (CX) collision cross sections of highly charged ions (HCIs) and neutral atoms play a crucial role in fundamental and applied physics. An experimental study on single- and double-electron capture (SEC and DEC) processes for C⁴⁺(1s²) - He in collision energy between 2.3 and 33.3 keV u^-1 was performed via the cold-target recoil-ion momentum spectroscopy (COLTRIMS) apparatus at Fudan University. For SEC, the measurement showed an increasing tendency of electrons to be captured into higher-n states because of the opening of new CX channels with the rise of impact energy. In the measurement of DEC, a similar gradually increasing trend was recognized from the contribution of highly excited states. A turning point was identified where the impact energy is around 20 keV u^-1, beyond which the contribution of DEC to the ground state 2s² became minimal, while that from higher excited states became dominant. In addition, we also found significant contributions to DEC arising from asymmetric configurations. This might be linked to the vital role that electron-electron interaction plays in the formation of asymmetric configurations, as described by the two-active-electron semiclassical atomic-orbital close-coupling theory. These observations serve as experimental benchmarks for theoretical calculations and plasma spectral analysis in various environments.