1. Biometrology, Chemical and Biological Sciences Department, National Physical Laboratory, Teddington, Middlesex, UK
2. Manchester Institute of Biotechnology and Department of Chemistry, The University of Manchester, 131 Princess Street, Manchester, UK
3. Central Laser Facility, STFC Rutherford Appleton Laboratory, Harwell Campus, Didcot, UK
4. Department of Chemistry and York Biomedical Research Institute, University of York, UK