1. Department of Chemistry, Imperial College of Science, Technology and Medicine, Exhibition Road, London SW7 2AY, U.K.; Central Laser Facility, CLRC Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0QX, U.K.; Department of Physics, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, U.K.; and Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, India