1. Nano and Molecular Systems Research Unit, Faculty of Science, University of Oulu, P.O. Box 3000, FI-90014 Oulu, Finland
2. Universität Kassel, Institut für Physik und CINSaT, Heinrich-Plett-Straße 40, 34132 Kassel, Germany
3. MAX IV Laboratory, Lund University, P.O. Box 118, SE-22100 Lund, Sweden
4. Institute of Physics, University of Tartu, W. Ostwaldi 1, EE-50411 Tartu, Estonia