1. Contribution from the FOM-Institute for Plasmaphysics Rijnhuizen, Edisonbaan 14, NL-3439 MN Nieuwegein, The Netherlands, German Cancer Research Center, Im Neuenheimer Feld 580, D-69120 Heidelberg, Germany, Chemistry Department, Physical and Theoretical Chemistry Laboratory, Oxford University, Oxford OX1 3QZ, U.K., and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany