1. Contribution from the Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, Institut für Physikalische Chemie, Lehrgebiet A, Universität Hannover, D-30167 Hannover, Germany, Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, and Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109