1. Institute of Physics of the Czech Academy of Sciences, 182 21 Prague 8, Czech Republic
2. Ioffe Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia
3. Experimentelle Physik 2, Technische Universität Dortmund, D-44221 Dortmund, Germany
4. State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China