1. a Hahn-Meitner-Institute , Glienickerstr. 100, D-1000, Berlin 39, Germany
2. b Division of Applied Physics, CSIRO, and Research School of Physical Sciences, Institute of Advanced Studies, Australian National University , GPO Box 4, Canberra, ACT 2601, Australia
3. c Depto. de Fisica, Universidad Autonoma Metropolitana, Iztapalapa , Mexico DF., Mexico
4. d Fernuniversität Hagen , Elberfelder Str. 95, D-5800, Hagen, Germany
5. e Ceskoslovesnko Akademia Vod, Ustav Jaderne Fyzyky , 25068, Rez near Prague, CSFR
6. f University of Minsk , Leninskii Prospect 2, 220080, Minsk, Byelorussia