1. Departments of Chemistry and Physics, College of Natural Sciences; University of Puerto Rico; PO Box 70377 San Juan Puerto Rico 00936-8377 USA
2. Department of Mining and Materials Engineering; McGill University; 3610 University Street Montréal Québec Canada H3A 0C5
3. Instituto Tecnológico de Cancún; Kabah Km 3 Cancún Quintana Roo 77500 México