1. Department of Physics, P.O. Box 48, and Laboratory of Polymer Chemistry, Department of Chemistry, P.O. Box 55, University of Helsinki, FI-00014 Helsinki, Finland, Institute of Physical Chemistry, University of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria, and Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093-0356