1. Departments of Chemistry and Physics, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejon 305-401, Republic of Korea, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, and Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210