1. FOM Institute for Plasma Physics “Rijnhuizen”, Edisonbaan 14, NL-3439 MN Nieuwegein, The Netherlands, Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, D-12489 Berlin, Germany, Department of Molecular and Laser Physics, University of Nijmegen, Toernooiveld, NL-6525 ED Nijmegen, The Netherlands, and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany