[1] Departamento de Física, Univ. Federal de São Carlos, 13565-905 Sao Carlos, Sao Paulo, Brazil
[2] P. N. Lebedev Physics Institute, Leninsky Prospect 53, 117924 Moscow, Russia
[3] P. N. Lebedev Physics Institute, Moscow Inst. of Phys. and Technology, Dolgoprudny, Moscow region 141700, Russia