1. Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ U.K., Davy Faraday Research Laboratory, Royal Institution of Great Britain, 21 Albermarle Street, London, W1X 4BS U.K., and University Chemical Laboratory, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, U.K.