1. Institute of Physics, Solid Surfaces Analysis Group, Chemnitz University of Technology, D-09107, Chemnitz, Germany, Center for Material Elaboration & Structural Studies, 29, Rue Jeanne Marvig, B.P. 94347, 31055 Toulouse Cedex, France, and Technische Universität Kaiserslautern, FB Chemie, Erwin-Schrödinger-Strasse Geb. 54, D-67663 Kaiserslautern, Germany