1. Department of Physical Chemistry 1, Lund University, P.O. Box 124, S-221 00 Lund, Sweden, Institute of Chemistry, Eötvös Loránd University, Budapest 112, P.O. Box 32, H-1518 Hungary, Department of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning Kristinas Väg 51, S-100 44 Stockholm, Sweden, and Institut Laue-Langevin, BP 156, 38042 Grenoble Cedex 9, France