1. Department of Chemistry, University of Jyväskylä, P.O. Box 35, FIN-40351 Jyväskylä, Finland, Department of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands, and Facoltà di Scienze MM. FF. NN., Università di Verona, Strada Le Grazie-Ca' Vignal, 37134 Verona, Italy