1. Laboratory of Physical Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland
2. Van Swinderen Institute for Particle Physics and Gravity, University of Groningen, Nijenborgh 4, 9747 AG Groningen, Netherlands
3. VTT Technical Research Centre of Finland Ltd., Centre of Metrology MIKES, P.O. Box 1000, Espoo FI-02044 VTT, Finland