1. Institut für Angewandte Physik, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, United Kingdom, Chemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford, OX1 3TA, United Kingdom, Department of Physics, University of Bath, Bath, BA2 7AY, United Kingdom, and SRS, Daresbury, Warrington, Cheshire, WA4 4AD, United Kingdom