1. Solid Surface Analysis, Institute of Physics, Chemnitz University of Technology, Reichenhainer Strasse 70, Chemnitz 09126, Germany
2. Department of Applied Physics, Hebrew University of Jerusalem, Jerusalem 91905, Israel
3. Center for Nanoscience and Nanotechnology, Hebrew University of Jerusalem, Jerusalem 91905, Israel
4. Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, Warszawa 02-668, Poland