1. Dipartimento di Fisica e Astronomia; Università di Catania, Via Santa Sofia 64; 95123 Catania Italy
2. CNR-IMM UOS Università (MATIS); Consiglio Nazionale delle Ricerche, Via Santa Sofia 64; 95123 Catania, Italy & INFN, Sezione di Catania, Via Santa Sofia 64; 95123 Catania Italy
3. Centre for Theoretical Atomic; Molecular and Optical Physics; School of Mathematics and Physics, Queen's University Belfast; Belfast BT7 1NN United Kingdom
4. Low Temperature Laboratory; Department of Applied Physics; Aalto University School of Science; P.O. Box 15100 FI-00076 Aalto Finland