1. Centre de Physique Moléculaire Optique et Hertzienne, Université Bordeaux I and CNRS (UMR 5798), 351 cours de la Libération, 33405 Talence Cedex, France, Centre de Recherche Paul Pascal, CNRS, Avenue Schweitzer 33600 Pessac, France, and Department of Chemistry, Smalley Institute for Nanoscale Science and Technology and Center for Biological and Environmental Nanotechnology, Rice University, Houston, Texas 77005