1. L.I.M.H.P.-C.N.R.S., Institut Galilée, Université Paris Nord, 99 Av. J. B. Clément, 93430 Villetaneuse, France, and ICMCB-CNRS UPR 9048, Université Bordeaux I, 87 Av du Dr A. Schweitzer, F 33608 PESSAC Cedex, France, and Kazan State Technological University, 68 K. Marx Street, Kazan, 420015 Tatarstan, Russia