1. Institute of Inorganic Chemistry, University of Fribourg, Pérolles, 1700 Fribourg, Switzerland, Section of Physics, University of Genève, Quai Ernest-Ansermet 24, 1211 Genève 4, Switzerland, and Institute of Chemistry, University of Neuchâtel, Avenue de Bellevaux 51, 2000 Neuchâtel, Switzerland