1. Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, U.K.
2. Central Laser Facility, Science and Technology Facilities Council, Research Complex at Harwell, Didcot, Oxfordshire OX11 0FA, U.K.
3. Rutherford Appleton Laboratory, Research Complex at Harwell, Didcot, Oxfordshire OX11 0FA, U.K.
4. Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, RG4 9NH, U.K.
5. Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy
6. Istituto di Fotonica e Nanotecnologie, Consiglio Nazionale delle Ricerche, Piazza Leonardo da Vinci, I-20133 Milano, Italy