1. Department of Chemistry, Nankai University, Tianjin 300071, P.R. China
2. Department of Physics, Nankai University, Tianjin 300071, P.R. China
3. Department of Chemistry, Graduate School of Science, the University of Tokyo, Tokyo 113-0033, Japan
4. Rigaku Corporation, Matsubara-Cho 3-9-12, Akishima, Tokyo 196-8666, Japan