1. Key Lab of Organic Optoelectronics and Molecular Engineering Department of Chemistry Tsinghua University Haidian District Beijing 100084 P. R. China
2. Beijing Synchrotron Radiation Facility Institute of High Energy Physics Chinese Academy of Sciences 19B Yuquan Road, Shijingshan District Beijing 100049 P. R. China
3. School of Physical Sciences University of Chinese Academy of Sciences 19B Yuquan Road, Shijingshan District Beijing 100049 P. R. China