1. Experimental Physics VI, Julius-Maximilians-University of Würzburg, D-97074 Würzburg, Germany
2. Huygens Laboratory, Department of Molecular Physics, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands
3. Bavarian Centre for Applied Energy Research (ZAE Bayern), D-97074 Würzburg, Germany