1. Institut für Chemie und Biochemie, Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany, Institute of Macromolecular Compounds, Russian Academy of Science, Bolshoj pr., 31, St.Petersburg, Russia, and V. A. Fock Institute of Physics, St.Petersburg State University, Ulyanovskaja 1, 198504 Peterhof, Russia