1. Department of Physics and Astronomy, Aarhus University, Ny Munkegade, DK-8000 Aarhus C, Denmark
2. Department of Physics, Stockholm University, S-10691 Stockholm, Sweden
3. Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP), Boulevard Henry Becquerel, F-14070, Caen Cedex 5, France