1. Department of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, U.K., Chemistry Department, Brookhaven National Laboratory, P.O. Box 5000, Upton, New York 11973-5000, Laboratory of Chemical and Biological Dynamics, Center for Research on Molecular Electronics and Photonics, University of Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium, and Optical Sciences Center, University of Arizona, Tucson, Arizona 85721