1Laboratoire Francis Perrin (CNRS-URA-2453), DSM/DRECAM/ Service des Photons, Atomes et Molécules, C.E.A. Saclay, F-91191 Gif-sur-Yvette Cedex, France
2 Department of Molecular and Laser Physics, University of Nijmegen Toernooiveld 1, 6525ED Nijmegen, Pays-bas