Double electron excitation in He ions interacting with an aluminum surface

Observation of Auger electron emission from Al under the impact of He ions, at energies below the excitation threshold of Al-Al collisions, gives evidence for charge transfer in He-Al collisions. Excitation of 2p-shell electrons in Al is produced by vacancy transfer, following double excitation of He by promotion of its 1s level. Studies of ion scattering from surfaces currently neglect the double excitation of He. We discuss implications on the debate about electronic excitations and energy deposition, which is relevant to many fields, since helium ions are applied in the characterization and imaging of materials