1. Dept of Chemistry, The University of Reading, Reading RG6 6AD, U.K.
2. Centre for Advanced Microscopy, The University of Reading, Reading RG6 6AF, U.K.
3. Physical Chemistry 1, Lund University, S-221 00 Lund, Sweden
4. The Biocentre, The University of Reading, Reading RG6 6AS, U.K.