1. Fakultät Physik, Technische Universität Dortmund, D-44221 Dortmund, Germany
2. Institute of Physical Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria
3. Department of Physics, Sapienza Università di Roma, I-00185 Rome, Italy
4. Department of Physics, Stockholm University, SE-106 91 Stockholm, Sweden