1. Institute of Physics, University of Potsdam, Am Neuen Palais 10, D-14469 Potsdam, Germany, Institute of Ion Beam Physics and Materials Research, Bautzner Landstrasse 128, D-01328 Dresden, Germany, Department of Physics, Humboldt University Berlin, Newtonstrasse 15, D-12489 Berlin, Germany, Macromolecular Chemistry, University of Wuppertal, Gauss-Strasse 20, D-42097 Wuppertal, Germany, and Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands