Subshell-selective electron capture in collisions of C⁴⁺, N⁵⁺, O⁶⁺ with H, H₂ and He

By means of optical spectroscopy the authors have determined absolute subshell-selective single electron capture cross sections sigma _nl for collisions of C⁴⁺, N⁵⁺ and O⁶⁺ with atomic and molecular hydrogen and helium at impact velocities between 0.1 and 0.5 au. The absolute sensitivity calibration of the monochromators is discussed briefly. In addition they have measured the total single electron capture cross section sigma _t by means of charge state analysis, for atomic and molecular hydrogen as targets. In all cases it is found that one or two n shells are dominantly populated and that the total cross section is only weakly dependent on the impact velocity, in contrast to the subshell-selective cross sections sigma _nl. The main features of the results obtained are qualitatively explained by means of simple diabatic potential diagrams and the classical overbarrier transition model. For several systems a detailed comparison can be made with recent theoretical calculations, showing satisfactory agreement.