1. Swiss Federal Institute of Technology, ETH-ITO, Grabenstrasse 3, 8952 Schlieren, Switzerland, Department of Physics, Kent State University, P.O. Box 5190, Kent, Ohio 44242-0001, Institute of Applied Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland, and Department of Chemistry, Clarkson University, Potsdam, New York 13699-5814