1. Leiden Institute of Chemistry, Gorleaus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Institute for Plasma Physics Rijnhuizen, P.O. Box 1207, 2430 BE Nieuwegein, The Netherlands, and FOM-Institute for Atomic and Molecular Physics, Kruislaan 407 1098 SJ Amsterdam, The Netherlands