1. Department
of Chemistry, University College London, London WC1H 0AJ, U.K.
2. European Synchrotron Radiation Facility, BP 220, 38043 Grenoble, Cedex 9,
France
3. School of Chemistry, University of Southampton, Southampton SO17 1BJ, U.K.
4. Institut Laue-Langevin, BP 156, 38042 Grenoble, Cedex 9, France