1. Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wrocław 2, P.O. Box 937, Poland, Universite Pierre et Marie Curie, Laboratoire d'Optique des Solides, CNRS−UMR 7601, 4, place Jussieu, 75252 Paris Cedex 05, France, and Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland