1. Institut für Physik, Johannes Gutenberg-Universität, 55099 Mainz, Germany.
2. Fachbereich Physik, Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany.
3. Institute for Theoretical Atomic, Molecular and Optical Physics, Harvard-Smithsonian Center of Astrophysics, Cambridge, MA 02138, USA.
4. Department of Physics, Boston University, Boston, MA 02215, USA.
5. Physics Department, Harvard University, Cambridge, MA 02138, USA.