1. Université Paris 7-Denis Diderot, ITODYS, UMR CNRS 7086, case 7090, 2 place Jussieu, F-75251 Paris Cedex 05, France, Central Michigan University, Department of Physics, Mt. Pleasant, Michigan 48859, and Université de Toulouse, INSA, LPCNO, UMR CNRS 5215, 135 av. de Rangueil, 31077 Toulouse Cedex 4, France