1. L. P. B. C. (CNRS UMR 7033), Université Pierre et Marie Curie, 4 Place Jussieu, Case 138, F-75252 Paris Cedex 05, France, Institute of Physics, Charles University, Ke Karlovu 5, CZ-12116 Prague 2, Czech Republic, and Department of Physical and Macromolecular Chemistry, Charles University, Hlavova 2030, CZ-12840 Prague 2, Czech Republic