1. FOM-Institute for Plasmaphysics Rijnhuizen, Edisonbaan 14, NL-3439 MN Nieuwegein, The Netherlands, Department of Molecular and Laser Physics, University of Nijmegen, Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands, and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany