1. Physical Chemistry, University of Mainz, Jakob-Welder-Weg 11, 55128 Mainz, Germany, Center for Applied Photonics, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany, and Institute for Synchrotron Radiation, Forschungszentrum Karlsruhe, Postfach 3640, D-76021 Karlsruhe, Germany