By means of photon emission spectroscopy the authors have studied state selective one-electron capture and target-ion excitation in collisions of He²⁺ with He. The collision energy has been varied from 1 to 75 keV amu^-1. Four-body classical trajectory Monte Carlo calculations have been performed in the energy range of 50-300 keV amu^-1. In the energy range where experiment and theory overlap there is in general fair agreement, although the l-distributions within a principal quantum shell exhibit differences. At energies below 50 keV amu^-1 the experiments confirm the results of atomic orbital calculations, presented in an accompanying paper by Fritsch (1994). Combining all the experimental and theoretical data the authors have determined a reliable cross section data base for use in neutral helium beam based plasma diagnostics on large tokamaks.