1. Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, SE-412 96 Göteborg, Sweden
2. Centre for Theoretical Atomic, Molecular and Optical Physics, Queen's University Belfast, Belfast BT7 1NN, United Kingdom
3. Dipartimento di Fisica ``Aldo Pontremoli,'' Università degli Studi di Milano, I-20133 Milano, Italy