1. Contribution from the Departments of Physics and Chemistry, University of Durham, Durham DH1 3LE, U.K., Service de Chimie des Materiaux Nouveaux, Université de Mons Hainaut, B-7000 Mons, Belgium, Physics Department, University of Antwerp-UIA, Universiteitsplein 1, B-2610 Antwerp, Belgium, and The Beckman Institute, California Institute of Technology, Pasadena, California 91125