[1] INFM, CNR, TASC Lab, I-34012 Trieste, Italy
[2] Univ Paris 06, Lab Chim Phys Mat & Rayonnement, UMR 7614, F-75005 Paris, France
[3] INFM, CNR, Natl Res Ctr, I-41100 Modena, Italy
[4] CNISM, I-00146 Rome, Italy
[5] Univ Roma 3, Dipartimento Fis, I-00146 Rome, Italy
[6] Sincrotrone Trieste, I-34012 Trieste, Italy
[7] European Synchrotron Radiat Facil, F-38042 Grenoble 9, France