1. Department of Chemistry, University of Sheffield, Sheffield S3 7HF, U.K.
2. School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K.
3. Central Laser Facility, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, U.K.