1. FOM-Institute for Plasmaphysics Rijnhuizen, Edisonbaan 14, NL-3439 MN Nieuwegein, The Netherlands, Department of Molecular and Laser Physics, University of Nijmegen, Toernooiveld, NL-6525 ED Nijmegen, The Netherlands, Laboratoire de Chimie Physique, CNRS-UMR-8000, Université de Paris XI, Bâtiment 350, 91405 Orsay Cedex, France, and Department of Chemistry, University of Georgia, Athens, Georgia 30602