1. Department of Chemistry, Stanford University, Stanford, California 94305, Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, Laboratoire de Chimie Inorganique, URA CNRS 420, Institut de Chimie Moléculaire d'Orsay, Université de Paris-Sud, 91405 Orsay, France, and Max-Planck-Institut für Strahlenchemie, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany