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