1. Contribution from the Department of Physical Chemistry and the Farkas Center for Light Induced Processes, The Hebrew University, Givat Ram, Jerusalem 91904, Israel, the Institut für Nanotechnologie, Forschungszentrum Karlsruhe, Postfach 3640, 76021 Karlsruhe, Germany, and the Institut für Anorganische Chemie, Universität Karlsruhe, Engesserstrasse Geb. 30.45, D-76128 Karlsruhe, Germany