1. Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2Y, U.K., Instituto de Física, Universidad de Guanajuato, León 37150, México, and Research and Technology, ICI Chemicals and Polymers, PO Box 8, The Heath, Runcorn, Cheshire WA7 4QD, U.K.