1. Department of Chemistry, University of Jyväskylä, P.O. Box 35, FIN-40351 Jyväskylä, Finland, Division of Physics and Astronomy of the Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands, and Botanisches Institut der Universität München, Menzinger Strasse 67, D-80638 München, Germany