1. Contribution from the Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Heverlee-Leuven, Belgium, Department of Chemistry, Faculty of Science, Nara Women's University, Nara 630, Japan, Department of Applied Physics, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan, and Institut für Physikalische und Theoretische Chemie, Humboldt-Universität, Bunsenstrasse 1, D-10117 Berlin, Germany