1. Department of Physics; Liaoning University; Shenyang 110036 P. R. China
2. Beijing National Laboratory for Condensed Matter Physics; Institute of Physics Chinese Academy of Science; Beijing 100190 P. R. China
3. State Key laboratory of Molecular Reaction Dynamics; Dalian Institute of Chemical Physics Chinese Academy of Science; Dalian 116023 P. R. China
4. College of Science; Northeast Forestry University; Harbin 150040 P. R. China
5. Department of Chemical Physics; Lund University; Box 124 Lund 22100 Sweden