1. Department of Chemistry and Biochemistry, Ludwig Maximilian University Munich, Butenandtstrasse 5-13 (Haus D), D-81377 München, Germany, Groupe des Sciences Moléculaires, Université Bordeaux 1, ICMCB, CNRS, 87 Av. du Doc. A. Schweitzer, F-33608 Pessac, France, and Department of Physics, Technical University Munich, James-Franck-Strasse 85747 Garching, Germany