1. Department of Physics, Free University Berlin, Arnimallee 14, D-14195 Berlin, Germany, Semenov Institute of Chemical Physics, Kosygina str. 4, 119991 Moscow, Russia, Institute of Chemical Kinetics and Combustion, Institutskaya str. 3, 630090 Novosibirsk, Russia, and Max-Planck-Institute for Medical Research, Jahnstrasse 29, D-69120 Heidelberg, Germany