1. TU Wien, Institute of Applied Physics, Vienna 1040, Austria
2. University of Vienna, Faculty of Physics, Vienna 1090, Austria
3. Trinity College Dublin, AMBER & School of Chemistry, Dublin D2, Ireland
4. TU Wien, Institute for Theoretical Physics, Vienna 1040, Austria
5. Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Dresden 01328, Germany