1. Institut für Ionenphysik und Angewandte Physik and Center of Molecular Biosciences Innsbruck, Universität Innsbruck, Technikerstr. 25, A-6020 Innsbruck, Austria, Department of Physics, King’s College London, The Strand, London WC2R 2LS, United Kingdom, and Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom