1. Service des Photons, Atomes, et Molécules, Centre d'Etudes de Saclay, 91191 Gif-sur-Yvette, France
2. Department of Physics, Lund Institute of Technology, P.O. Box 118, S-221 00 Lund, Sweden
3. Laser Program, Lawrence Livermore National Laboratory, P.O. Box 808, L493, Livermore, CA 94550, USA