1. Institut für Physik, Technische Universität Ilmenau, D-98693Ilmenau, Germany
2. Peter Grünberg Institut, Forschungszentrum Jülich, D-52425Jülich, Germany
3. II. Physikalisches Institut, Universität zu Köln, D-50923Cologne, Germany
4. Fachbereich Physik, Universität Konstanz, D-78457Konstanz, Germany
5. Department of Theoretical Solid State Physics, Institute for Solid State Physics and Optics, Wigner Research Center for Physics, H-1121Budapest, Hungary
6. ELKH-SZTE Reaction Kinetics and Surface Chemistry Research Group, University of Szeged, H-6720Szeged, Hungary
7. Department of Theoretical Physics, Institute of Physics, Budapest University of Technology and Economics, H-1111Budapest, Hungary