1. Department of Physics, SUPA, University of Strathclyde, Glasgow G4 0NG, U.K.
2. STFC Central Laser Facility, Research Complex at Harwell, Rutherford Appleton Laboratory, Harwell Campus, Didcot OX11 0QX, U.K.
3. Department of Chemistry and York Biomedical Research Institute, University of York, Heslington, York YO10 5DD, U.K.