1. Istituto per i Processi Chimico-Fisici, Consiglio Nazionale delle Ricerche, via La Farina 237, 98123 Messina, Italy, Hahn-Meitner Institut, Glienicker Str. 100, D-14109 Berlin, Germany, QUILL Research Centre and School of Chemistry, Queen's University, Belfast BT9 5AG, Northern Ireland, Institut Laue Langevin, 6, rue J. Horowitz, BP 156, 38042 Grenoble Cedex 9, France, and Institut für Festkörperforschung, Forschungs Zentrum Jülich GmbH, D-52425 Jülich, Germany