1. Department of Chemistry, Surface Chemistry, Drottning Kristinas väg 51, Royal Institute of Technology, SE-100 44 Stockholm, and YKI, Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden
2. Institut Laue-Langevin (ILL), DS/LSS, 6, rue Jules Horowitz, B.P. 156, 38042 Grenoble Cedex, France