1. Department of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, U.K., Department of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, U.K., 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