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 sc attering angle, for single- and double-electron capture processes in collis ions of C5+ ions with He at laboratory energies between 0.5 and 50 keV/u, h ave been studied experimentally by means of a high-resolution cold-target r ecoil-ion momentum spectroscopy. The translational energy spectra for singl e-electron capture show that capture into the C4+ (a = 3) state is the domi nant reaction channel observed over the entire collision energy region stud ied, in agreement with close-coupling calculations. However, capture into n = 2 and n greater than or equal to 4 states is only significant at E great er than or equal to 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 predi ction. The autoionizing double-electron-capture (transfer ionization) spect rum clearly shows that C3+ (2l,2l') states are populated at the lowest impa ct energy, whereas contributions from capture into C3+ (2l,n'l') states wit h n' greater than or equal to 3 increase as the impact energy is increased and become the dominant process at E = 35 keV/u. State-selective and differ ential cross sections have also been measured and compared with the close-c oupling and multichannel Landau-Zener calculations. [S1050-2947(99)08009-9] .