State-selective electron capture in (0.5–50)-keV/u C⁵⁺-He collisions studied by cold-target recoil-ion momentum spectroscopy

Cross sections, differential in translational energy gain and projectile scattering angle, for single- and double-electron capture processes in collisions of C⁵⁺ ions with He at laboratory energies between 0.5 and 50 keV/u, have been studied experimentally by means of a high-resolution cold-target recoil-ion momentum spectroscopy. The translational energy spectra for single-electron capture show that capture into the C⁴⁺ (n=3) state is the dominant reaction channel observed over the entire collision energy region studied, in agreement with close-coupling calculations. However, capture into n=2 and n>~4 states is only significant at E>~4 keV/u. Transfer excitation processes contribute only very weakly to the total cross sections for single-electron capture (2–4 %). This contribution is considerably smaller than the close-coupling prediction. The autoionizing double-electron-capture (transfer ionization) spectrum clearly shows that C³⁺ (2l,2l^′) states are populated at the lowest impact energy, whereas contributions from capture into C³⁺ (2l,n^′l^′) states with n^′>~3 increase as the impact energy is increased and become the dominant process at E=35 keV/u. State-selective and differential cross sections have also been measured and compared with the close-coupling and multichannel Landau-Zener calculations.