1. Department of Physics “E. Fermi”, University of Pisa, largo B. Pontecorvo 3, 56127 Pisa, Italy; CNR, INFM, polyLab, largo B. Pontecorvo 3, 56127 Pisa, Italy; and Department of Chemistry and Industrial Chemistry, University of Pisa, via Risorgimento 35, 56124 Pisa, Italy