1. Department of Applied Physics, University of Tokyo, and Cooperative Excitation Project of ERATO (JST), 7-3-1, Hongo Bunkyo-ku, Tokyo 113-8656
2. Chemistry Department, University of California, Santa Barbara, California 93106, U.S.A.
3. Department of Physics, University of Science & Technology of China, Hefei, Anhui, 230026, P.R. China