1. a Electron Microscopy Centre, Chemistry Department , University of Glasgow , Glasgow , G12 8QQ , Scotland
2. c ICI Imagedata, Brantham, Manningtree , Essex CO11 1NL, England
3. d Sri Guru Rai College , Dehra Dun , India
4. b Imperial Chemical Industries plc, Electronics Group , Runcorn , Cheshire , WA7 4QE , England