1. Gorlaeus Laboratories, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
2. Laboratory of Physical Chemistry, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
3. Philips Research Laboratories, Eindhoven (WA11), Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands