1. Van der Waals-Zeeman Institute, Institute of Physics, University of Amsterdam, 1098 XH Amsterdam, The Netherlands
2. Department of Molecular Spectroscopy, Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
3. SKF Engineering & Research Center, P.O. Box 2350, 3430DT Nieuwegein, The Netherlands
4. Laboratoire de Physique Statistique de l'Ecole Normale Superieure, 24 rue Lhomond, 75231 Paris Cedex 05, France