1. State Key Laboratory of Crystal Materials; Shandong University; Jinan 250100 P.R. China
2. Beijing Synchrotron Radiation Facility; Institute of High Energy Physics, Chinese Academy of Sciences; Beijing 100049 P.R. China
3. School of Physics, Shandong University; Jinan 250100 P.R. China
4. Department of Chemistry; North Carolina State University; Raleigh NC 27695-8204 USA