1. Institute of Molecular and Atomic Physics, National Academy of Sciences, F. Skaryna ave. 70, 220072 Minsk, Belarus
2. National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32304, USA
3. Institut für Physik, Universität Dortmund, D-44221 Dortmund, Germany
4. Bogolubov Laboratory of Theoretical Physics, Joint Institute for Nuclear Research, 141980 Dubna, Moscow Region, Russia