[1] Byelarussian Acad Sci, Inst Atom & Mol Phys, Minsk 220072, BELARUS
[2] Byelarussian Acad Sci, BI Stepanov Phys Inst, Minsk 220072, BELARUS
[3] Polish Acad Sci, Inst Low Temp & Struct Res, PL-50950 Wroclaw 2, Poland
[4] SI Vavilov State Opt Inst, St Petersburg 199034, Russia