1. Contribution from the Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, Institut für Experimentelle Physik, Universität Bremen, 28359 Bremen, Germany, the Department of Chemistry, City University of New York, Hunter College, New York 10021, and the Department of Chemistry, University of Puerto Rico, Rio Piedras Campus, PO. Box 23346, San Juan, Puerto Rico 00931-3346