1. Contribution from the Department of Chemistry, University of Georgia, Athens, Georgia 30602, FOM Institute for Plasmaphysics Rijnhuizen, Edisonbaan 14, NL-3439 MN Nieuwegein, The Netherlands, Combustion Research Facility, Sandia National Laboratory, Mail Stop 9055, P.O. Box 969, Livermore, California 94550, and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany