1. Instituto de Física, UNAM, Apartado Postal 20-364, México D. F., México, and Texas Materials Institute, Department of Chemical Engineering, and Center for Nano and Molecular Science and Technology, University of Texas at Austin, Austin, Texas 78712-1062, and Centro Nuclear Dr Nabor Carrillo Flores, ININ, Km. 36.5 Carr-Fed. México-Toluca, CP 52045, México, and Centro de Química, IVIC, Apartado 21827, Caracas 1020A, Venezuela