1. Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei, Taiwan, ROC, Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan, Department of Physics, National Taiwan University, Taipei, Taiwan, Department of Chemistry, University of Rochester, Rochester, New York 14627, and Department of Chemical Engineering, National Tsing-Hwa University, Hsinchu, Taiwan