1. Department
of Chemistry and Pharmacy, University of Sassari, 07100 Sassari, Italy
2. Université libre de Bruxelles (ULB), Nonlinear
Physical Chemistry Unit, Faculté
des Sciences, CP231, 1050 Brussels, Belgium
3. Nonlinear
Physics Group, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain