1. Fritz-Haber-Institut der Max-Planck-Gesellschaft, Department of Chemical Physics, Faradayweg 4-6, D-14195 Berlin, Germany, University of Wroclaw, Institute of Experimental Physics, Pl. Maxa Borna 9, PL-50-204 Wroclaw, Poland, and Universita degli Studi Milano-Bicocca, Dipartimento di Scienza dei Materiali, via R. Cozzi, I-53-20125 Milano, Italy