1. School of Chemistry, Physics, and Environmental Science, University of Sussex, Brighton BN1 9QJ, U.K., Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K., Institut Laue Langevin, BP 156, 38042 Grenoble, France, and Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan