1. Contribution from the Laboratoire de Photonique Quantique et Moléculaire-UMR CNRS 8537, Ecole Normale Supérieure de Cachan, 61 avenue du Président Wilson, 94235 Cachan Cedex, France, Department of Chemistry, University of Rochester, Rochester, New York 14627, and Department of Chemistry and Materials, University of California, Santa Barbara, California 93106