1. Raman Spectroscopy Group, University Analytical Centre, Division of Chemical and Forensic Sciences, University of Bradford, West Yorkshire, BD7 1DP, U.K.
2. Cobalt Light Systems Ltd, Start Electron, Fermi Avenue, Harwell Science & Innovation Campus, Didcot, Oxfordshire, OX11 0QR, UKU.K.
3. Central Laser Facility, Science & Technology Facilities Council, Rutherford Appleton Laboratory, Oxfordshire, OX11 0QX, U.K.