1. Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warszawa, Poland, Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, 162 00 Prague 6, Czech Republic, Faculty of Mathematics and Physics, Charles University, 180 00 Prague 8, Czech Republic, and Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 02 Prague 6, Czech Republic