1. Department of Physics, Shanghai University, 99 Shangda Road, 200444 Shanghai, P. R. China
2. Department of Physics, Humboldt-Universität zu Berlin, 12489 Berlin, Germany
3. Institut für Chemie und Biochemie, Freie Universität Berlin, 14195 Berlin, Germany
4. MAX IV Laboratory, Lund University, 22100 Lund, Sweden