1. Organische Chemie III, Makromolekulare Chemie, Universität Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany, Digital Instruments, Robin Hill Road 112, California 93117, FOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands, and DUBBLE CRG, ESRF, BP 220, 38043 Grenoble, France