1. Department of Chemical Science, University of Catania, Viale A. Doria 6, I-95125 Catania, Italy
2. Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland
3. Department of Physics, University of Helsinki, P.O. Box 64, FI-00014 Helsinki, Finland
4. Department of Physics, Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, Pennsylvania 15213, USA