1. Department of Chemical and Biomolecular Engineering, Carbon Nanotechnology Laboratory, The Smalley Institute for Nanoscale Science and Technology, Department of Chemistry, and Department of Physics and Astronomy, Rice University, 6100 Main Street, Houston, Texas 77005, and Centre de Physique Moléculaire Optique et Hertzienne, Université Bordeaux 1 and CNRS, Talence F-33405, France