1. Contribution from the FOM-Institute for Plasmaphysics Rijnhuizen, Edisonbaan 14, NL-3439 MN Nieuwegein, The Netherlands, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany, and Chemistry Department, Case Western Reserve University, Cleveland, Ohio 44106