1. Laboratoire L.M.2.N, URA CNRS 1662, Université P. et M. Curie (Paris VI), BP 52, 4 place Jussieu, 75252 Paris Cedex 05, France, CEA-Saclay, DSM-DRECAM-SCM, 91191 Gif-sur-Yvette Cedex, France, and CEA-Saclay, DSM-DRECAM-SPEC, 91191 Gif-sur-Yvette Cedex, France