1. Department of Chemistry and Department of Physics, American University of Beirut, Bliss Street, P.O. Box 11-0236, Beirut, Lebanon, Department of Chemistry, Georgetown University, Box 571227, Washington, D.C. 20057-1227, and Laboratoire de Physico-Chimie de la Matière Condensée, Université Montpellier II, case courrier 003, 34095 Montpellier Cedex 5, France