Experimental evidence of transfer excitation in Ar6+-He collisions

High-resolution cold-target recoil-ion momentum spectroscopy (COLTRIMS) has been used to study single-electron capture processes in collisions of Ar6 ions with He at an impact energy of 9 keV and scattering angles between 0 and 3 mrad. The energy-gain spectra show that the capture occurs mainly int o the 4s state with a significant contribution involving capture into the 4 p state. Reaction channels associated with transfer excitation into the 3s3 p3d states of Ar5+ are also observed for the first time, accounting for 6(/-0.4)% of the total cross sections for single-electron capture. Experiment al results are found to be in good agreement with the two-electron atomic o rbital close-coupling calculations.