1. Contribution from the Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei 10764, Taiwan, Department of Chemistry, Tamkang University, Tamsui 25137, Taiwan, Department of Physics, Technical University Chemnitz-Zwickau, 09107 Chemnitz, Germany, and Department of Physics, National Taiwan University, Taipei 106, Taiwan