1. Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London, U.K. W1S 4BS, Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, U.K. CB2 1EW, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, U.K. CB2 3QZ, and Materials Research Laboratory, Pennsylvania State University, University Park, Pennsylvania, 16802