1. Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1S 4BS, U.K., Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K., and Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K.