1. Department of Materials and Interfaces, The Weizmann Institute of Science, 76100 Rehovot, Israel, Physics Department, 151 Le Conte Hall, University of California, Berkeley, California 94720, Chemistry Department, Aarhus University, Langelandsgade 140, DK-8000 Århus C, Denmark, Materials Research Department, Risø National Laboratory, DK-4000, Roskilde, Denmark, and ESRF, BP220, F-38043 Grenoble, Cedex, France