1. Laboratory of Thermochemistry, Chemical Sciences, Faculty of Health and Medical Sciences, University of Surrey, Guildford, GU2 7XH, U.K., Departamento de Física, Facultad de Ciencias Exactas, Universidad Nacional de La Plata and Institute IFLP (CONICET), C. C. 67, 1900 La Plata, Argentina, and Instituto Física de São Carlos, Universidade de São Paulo, C. P. 369, 13560 São Carlos (SP), Brazil.