1. School of Chemistry, University of Bristol, Bristol BS8 1TS, UK.
2. School of Physics, University of Bristol, Bristol BS8 1TL, UK.
3. Central Laser Facility, Science and Technology Facilities Council, Research Complex at Harwell, Rutherford Appleton Laboratory, Didcot OX11 0QX, UK.
4. Department of Chemistry, University of Toronto, Toronto, Ontario, M5S 3H6, Canada.