1. Institute of Applied Physics, University of Greifswald, Jahnstrasse 16, 17487 Greifswald, Germany, Institute of Physical Chemistry, University of Mainz, Welderweg 11, 55099 Mainz, Germany, Institute of Pharmacy, University of Greifswald, Jahnstrasse 17, 17487 Greifswald, Germany, and Institute of Solid State Physics, University of Jena, Helmholtzweg 5, 07743 Jena, Germany