1. Dipartimento
di Fisica, Università di Pisa, Largo B. Pontecorvo 3, I-56127, Pisa, Italy
2. CNR-IPCF, Institute for Chemical and Physical Processes, Largo B. Pontecorvo 3, I-56127, Pisa, Italy
3. Institute
of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland