1. Adolphe Merkle Institute; University of Fribourg, Chemin des Verdiers; 4 CH-1700 Fribourg Switzerland
2. Department of Physics; University of Cambridge; JJ Thompson Avenue CB3 0HE Cambridge UK
3. Swiss Light Source; Paul Scherrer Institute; CH-5232 Villigen Switzerland
4. Institute of Structural Physics; Technische Universität Dresden; 01062 Dresden Germany
5. Department of Chemistry; University of Cambridge; Lensfield Road CB2 1EW Cambridge UK
6. Department of Chemistry; Princeton University; Princeton NJ 08544 USA
7. Deutsches Elektronen-Synchrotron DESY; Notkestr. 85 22607 Hamburg Germany
8. Department of Physics; Universität Hamburg; Luruper Chaussee 149 22761 Hamburg Germany