1. Institut Laue-Langevin, B.P. 156, F-38042 Grenoble, France; Department of Chemical Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260-4200; and Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstrasse 40, D-24098 Kiel, Germany