1. Laboratory of Molecular Spectroscopy, Institute of Chemistry, Eötvös University, P.O. Box 32, H-1518, Budapest 112, Hungary, Laboratory of Theoretical Chemistry, Institute of Chemistry, Eötvös University, P.O. Box 32, H-1518, Budapest 112, Hungary, and Institut für Physikalische Chemie, Universität Mainz, D-55099 Mainz, Germany