1. School of Physics; Northwest University; Xi'an 710069 P. R. China
2. Key Laboratory of Applied Surface and Colloid Chemistry; National Ministry of Education; Shaanxi Engineering Lab for Advanced Energy Technology; School of Materials Science and Engineering; Shaanxi Normal University; Xi'an 710119 P. R. China
3. School of Physics and Information Technology; Shaanxi Normal University; Xi'an 710119 P. R. China
4. Dalian Institute of Chemical Physics; Chinese Academy of Sciences; Dalian 116023 P. R. China
5. Department of Chemistry; University of Nebraska-Lincoln; Lincoln NE 68588 USA