1. Atomic and Molecular Collisions Laboratory, CEFITEC, Department of Physics, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal
2. Department of Mathematics, Instituto Superior de Engenharia de Lisboa, R. Conselheiro Emídio Navarro 1, 1959-007 Lisboa, Portugal
3. Instituto de Fı́sica Fundamental, Consejo Superior de Investigaciones Cientı́ficas (CSIC), Serrano 113-bis, 28006 Madrid, Spain
4. Institut Lumière Matière, CNRS, Université Claude Bernard Lyon 1, 69622 Villeurbanne, France