1. Faculty of Physics, "Al. I. Cuza" University, Blvd. Carol I, no. 11, 700506 Iasi, Romania
2. Laboratoire de Physique des Lasers, Atomes et Molécules (UMR 8523), Université des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq cedex, France
3. Department of Physics, Technical "Gh. Asachi" University, Blvd. Mangeron, no. 64, 700029 Iasi, Romania
4. Faculty of Applied Sciences, Politehnica University of Bucharest, Department of Physics I, 313 Splaiul Independentei Street, 060042 Bucharest, Romania