1. Laboratory of Physical Chemistry, P.O. Box 55 (A.I. Virtasen aukio 1), FIN-00014 University of Helsinki, Finland, Department of Chemistry, University of Otago, P.O. Box 56, 9054 Dunedin, New Zealand, and Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom