1. Division of Synchrotron Radiation Research, Lund University, Box 118, 221 00 Lund, Sweden
2. Division of Atomic Physics, Lund University, Box 118, 221 00 Lund, Sweden
3. Institut Lumière Matière, UMR5306, Université Lyon 1-CNRS, 10 rue Ada Byron, 69622 Villeurbanne cedex, France