1. Department of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, S-106 91, Stockholm, Sweden
2. College of Physics and Electronics, Shandong Normal University, Jinan, Shandong 250014, P. R. China
3. National Synchrotron Radiation Laboratory and Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China