1. CEA-Valrhô Marcoule, DEN/DRCP/SCPS/LCA, Bât 399, BP 17171, 30207 Bagnols-sur-Cèze Cedex, France, Laboratoire LI2C (UMR 7612), Université P. M. Curie, Boîte No. 51, 4 Place Jussieu, 75252 Paris Cedex 05, France, and Department of Physics, POB 23343, University of Puerto Rico, Rio Pedras, Puerto Rico 00931-3343