1. Institut für Physikalische Chemie, Universität Göttingen, Tammannstr. 6, D-37077 Göttingen, Germany
2. Department of Chemistry, Technical University of Denmark, Kemitorvet 206, DK-2800 Kgs. Lyngby, Denmark
3. MAX-IV Laboratory, Lund University, P. O. Box 118, SE-22100 Lund, Sweden