[1] Univ Camerino, Sch Sci & Technol, Div Phys, I-62032 Camerino, MC, Italy
[2] Ist Nazl Fis Nucl, Sez Perugia, Perugia, Italy
[3] Univ Malta, Dept Phys, MSD-2080 Msida, Malta
[4] Queens Univ Belfast, Sch Math & Phys, Ctr Theoret Atom Mol & Opt Phys, Belfast BT7 1NN, Antrim, North Ireland