State-selective electron capture in (0.5–50)-keV/uC5+−Hecollisions studied by cold-target recoil-ion momentum spectroscopy

Abstract

Cross sections, differential in translational energy gain and projectile scattering angle, for single- and double-electron capture processes in collisions of ${\mathrm{C}}^{5+}$ 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 ${\mathrm{C}}^{4+}$ $(n=3)$ 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 $E>~4 \mathrm{k}\mathrm{e}\mathrm{V}/\mathrm{u}.$ 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 ${\mathrm{C}}^{3+}$ ${(2l,2l}^{\ensuremath{'}})$ states are populated at the lowest impact energy, whereas contributions from capture into ${\mathrm{C}}^{3+}$ ${(2l,n}^{\ensuremath{'}}{l}^{\ensuremath{'}})$ states with ${n}^{\ensuremath{'}}>~3$ increase as the impact energy is increased and become the dominant process at $E=35 \mathrm{k}\mathrm{e}\mathrm{V}/\mathrm{u}.$ State-selective and differential cross sections have also been measured and compared with the close-coupling and multichannel Landau-Zener calculations.