1. Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, Departament de Química Física, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain, Department of Physics, University of Oslo, P.O. Box 1048, Blindern, Norway, and Institut für Experimentalphysik V, Universität Bayreuth, 95440 Bayreuth, Germany