1. Foundation for Research and TechnologyHellas (FORTH), Institute of Electronic Structure and Laser, P.O. Box 1527, 711 10 Heraklion, Crete, Greece; Department of Chemistry, University of Athens, Panepistimiopolis, Zografou 15771, Athens, Greece; Max-Planck-Institut für Polymerforschung, Postfach 3148, D-55021 Mainz, FRG; and Department of Physics, Moscow State University, Moscow 117234, Russia