1. Department of Chemistry, Bagley Hall, Box 351700, University of Washington, Seattle, Washington 98195-1700, Department of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus, Denmark, Laboratory of Reaction Mechanisms, Ecole Polytechnique, 91128 Palaiseau, France, and German Cancer Research Center, D-69120 Heidelberg, Germany