1. Max-Planck-Institut für Kernphysik, Postfach 103980, D-69029 Heidelberg, Germany, Hasylab at Desy, Notkestrasse 85, D-22603 Hamburg, Germany, Chemical Research Center, Research Laboratory for Materials and Environmental Chemistry, Hungarian Academy of Sciences, Pusztaszeri út 59-67, H-1025 Budapest, Hungary, and Department of Theoretical Chemistry, Eötvös Loránd University, P. O. Box 32, H-1518 Budapest, Hungary