1. Physics Department, University of Trieste, via Valerio 2, I-34127 Trieste, Italy
2. IOM-CNR Laboratorio TASC, Area Science Park, S.S. 14 km 163.5, I-34149 Basovizza (Trieste), Italy
3. Abteilung Anorganische Chemie, Fritz-Haber Institut der Max-Planck Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany