1. School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.
2. Department of Physics, University of Bristol, Bristol BS8 1TS, U.K.
3. ISIS-CLRC, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, U.K.
4. School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
5. ILL, BP 156-X, F-38042 Grenoble Cedex, France