[1] Univ Bucharest, Fac Phys, Atomistilor St 405, Magurele 077125, Romania
[2] Alexandru Vlahuta Theoret Coll, Scoala Floreasca St 5, Bucharest 011641, Romania
[3] Inst Lasers Plasma & Radiat Phys INFLPR, Atomistilor St 409, Ilfov 077125, Romania
[4] Acad Romanian Scientists AOSR, Ilfov St 3, Bucharest 050045, Romania