1. Contribution from the FOM Institute for Plasma Physics “Rijnhuizen”, Postbus 1207, 3430 BE Nieuwegein, The Netherlands, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany, Chemistry Department, University of Florida, Gainesville, Florida 32611-7200, Chemistry Department, Case Western Reserve University, Cleveland, Ohio 44118