1. GSI Helmholtzzentrum für Schwerionenforschung, Planckstr. 1, D-64291 Darmstadt, Germany.
2. Institut für Angewandte Physik, TU Darmstadt, Schlossgartenstrasse 7, D-64289 Darmstadt, Germany.
3. Department of Physics, St. Petersburg State University, Oulianovskaya 1, Petrodvorets, St. Petersburg 198504, Russia.
4. Physics Department, Imperial College London, London, SW7 2BW, UK.