1. Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany
2. Helmholtz-Zentrum Geesthacht, Max-Planck-Straße 1, 21502 Geesthacht, Germany
3. Deutsches Elektronen-Synchrotron (DESY), D-22607 Hamburg, Germany
4. Physics Department, University of Hamburg, 20355 Hamburg, Germany