1. Polymer Science & Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden; Institut für Nukleare Festkörperphysik, Universität der Bundeswehr München, D-85577 Neubiberg, Germany; and Department of Subatomic and Radiation Physics, Universiteit Gent, Proeftuinstraat 86, B-9000 Gent, Belgium