1. Fritz-Haber-Institut der Max-Planck-Gesellschaft, Department of Inorganic Chemistry, Faradayweg 4-6, D-14195 Berlin, Germany, Department of Chemistry, Cardiff University, P. O. Box 912, Cardiff CF10 3TB, United Kingdom, Berliner Elektronenspeicherringgesellschaft für Synchrotronstrahlung (BESSY), Albert-Einstein-Strasse 15, D-12489 Berlin, Germany, and Lawrence Berkeley National Laboratory, Materials Science Division, Berkeley, California 94720