1. University of Konstanz, Department of Physics, 78 457 Konstanz, Germany
2. Univ. Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes), UMR 6226, F-35 000 Rennes, France
3. University of Mons, Laboratory for Chemistry of Novel Materials, Department of Chemistry, Place du Parc 20, B-7000 Mons, Belgium