1. The Royal Institution of Great Britain, 21 Albemarle Street, London W1S 4BS, U.K., Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ U.K., Brookhaven National Laboratory, Upton, New York 11973, and NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562