1. Department for Physical Chemistry, Friedrich-Schiller-University Jena, Helmholtzweg 4, D-07743 Jena, Germany, Institute for Physical High Technology, Albert-Einstein-Strasse 9, D-07745 Jena, Germany, and Institut für Physik, Technische Universität Ilmenau, Weimarer Strasse 32 (Faradaybau), D-98693 Ilmenau, Germany