1. Institute of Physical Chemistry, Christian-Albrechts-University of Kiel, Olshausenstr. 40, D-24098 Kiel, Germany, Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark, Institute Laue-Langevin, 6 rue Jules Horowitz, BP 156 - 38042 Grenoble Cedex 9, France, and Institute of Physical Chemistry, RWTH Aachen University, Templergraben 59, D-52056 Aachen, Germany