1. Contribution from the Department of Physics, Washington State University, 2710 University Drive, Richland, Washington 99352, W. R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, and J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic and Center for Complex Molecular Systems and Biomolecules, Dolejskova 3, 18223 Prague 8, Czech Republic