1. Department
of Physics, National University of Singapore, 2 Science Drive 3, Singapore, 117542
2. Department
of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543
3. Department
of Physics, La Trobe University, Victoria, Australia, 3086
4. Shanghai
Institute of Applied Physics, Chinese Academy of Sciences, P.O. Box 800-204, Shanghai, P. R. China, 201800