1. CNR-IPCF, Istituto per i Processi Chimico-Fisici, I-98158 Messina, Italy
2. MATIS CNR-IMM and Dipartimento di Fisica e Astronomia, Università di Catania, I-95123, Catania, Italy
3. Institute of Scientific Instruments of the CAS, v.v.i. Czech Academy of Sciences, 612 64 Brno, Czech Republic
4. H. H. Wills Physics Laboratory, University of Bristol, BS8 1TL Bristol, U.K.
5. Department of Physics and Astronomy, University College London, WC1E 6BT London, U.K.
6. Scuola Superiore di Catania, Università di Catania, I-95123 Catania, Italy