1. Atomic and Molecular Collisions Laboratory, CEFITEC, Department of Physics, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal
2. School of Physical Sciences, The Open University, Walton Hall, MK7 6AA Milton Keynes, United Kingdom
3. Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas (CSIC), Serrano 113-bis, 28006 Madrid, Spain
4. Institut Lumiére Matiére, Université Lyon, Université Claude Bernard Lyon 1, CNRS, 69622 Villeurbanne, France