1. Contribution from the Materials Research Laboratory, University of California, Santa Barbara, California 93106, LANSCE, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusung, Taejon 305-600, Korea, and Institut Lavoisier, UMR CNRS 173, Université de Versailles Saint Quentin, 45 avenue des Etats-Unis, 78035 Versailles Cedex, France