1. Department of Physical Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany.
2. Present address: Department of Physics, University of Fribourg, Chemin du Musée 3, 1700 Fribourg, Switzerland.
3. Department of Physics, University of Paderborn, Warburger Strasse 100, 33098 Paderborn, Germany.