1. Lennard Jones Laboratories, Keele University, Keele, ST5 5BG Staffordshire, U.K., BASF Nederland B.V. Innovatielaan 1, NL-8466 SN Nijehaske, The Netherlands, BASF Aktiengesellschaft, GKD/F-B001, D-67056 Ludwigshafen, Germany, BASF Corporation, 1609 Biddle Avenue, Wyandotte, Michigan 48192, HASYLAB at DESY, Notkestr. 85, 22603 Hamburg, Germany, and Physik Department LS E13, TU München, James-Franck-Str. 1, 85747 Garching, Germany