[1] Univ Geneva, Appl Phys Grp, CH-1211 Geneva 4, Switzerland
[2] Ecole Natl Super Chim Paris, Lab Chim Matiere Condensee Paris, CNRS, UMR 7574, F-75231 Paris 05, France
[3] Univ Parma, Dipartimento Chim Gen & Inorgan Chim Analit & Chi, I-43100 Parma, Italy
[4] Univ Verona, Dipartimento Sci & Tecnol, I-37134 Verona, Italy