1. Leiden Institute of Chemistry, University of Leiden, Einsteinweg 55, P. O. Box 9502, 2300 RA Leiden, The Netherlands, FOM Institute for Plasma Physics Rijnhuizen, Euratom-FOM Association, P. O. Box 1207, 3430 BE Nieuwegein, The Netherlands, and FOM Institute for Atomic and Molecular Physics AMOLF, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands