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+ (a = 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 greater than or equal to 4 states is only significant at E greater 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 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' 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 differential cross sections have also been measured and compared with the close-coupling and multichannel Landau-Zener calculations. [S1050-2947(99)08009-9].