1. Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei, Taiwan 106, Republic of China
2. Department of Physics, National Taiwan University, Taipei, Taiwan 106, Republic of China
3. Max-Planck-Institut für Strömungsforschung, Bunsenstrasse 10, D-37073 Göttingen, Germany
4. College of Science, Feng Chia University, Taichung, Taiwan 407, Republic of China