1. Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA.
2. School of Biological and Chemical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK.
3. Central Laser Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK.
4. Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853, USA.