1. IPREM/ECP, University of Pau, Hélioparc Pau Pyrénées, 2 av. Pierre Angot, 64053 Pau cedex 9, France; ICMMO/LPCES, University Paris XI, 15 av. Georges Clemenceau, 91405 Orsay cedex, France; and CEA/DRT/LITEN, 17 rue des Martyrs, 38054 Grenoble cedex 9, France