1. Contribution from the Centre for the Study of Excited States of Molecules, Huygens Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands, Gorlaeus Laboratories, Leiden Institute of Chemistry, P.O. Box 9502, 2300 RA Leiden, The Netherlands, and Max Planck Institut für Biochemie, Martinsried bei München, Germany