1. Division of X-ray Physics, Department of Physical Sciences, University of Helsinki, POB-64, FI-00014 University of Helsinki, Finland, Department of Physics, University of Durham, South Road, Durham DH1 3LE, United Kingdom, European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble, France, and Fachbereich Chemie, Bergische Universität Wuppertal, Gauss Strasse 20, D-42097 Wuppertal, Germany