Effect of u-g symmetry on direct and exchange excitations in ion-atom scattering

Electron capture processes in He + -He collisions are studied, using a time of flight technique. In particular, differential cross sections for both resonant electron capture and capture into excited states are measured. Cross sections for electron capture in n=2p states show an oscillatory behaviour which is out of phase with cross sections corresponding to direct n=2p excitation. This observation brings a new strong support to the interpretation of He + -He excitation in the keV energy range, through rotational coupling.