1. Department of Applied Quantum Physics, Kyushu University, Fukuoka 819-0395, Japan
2. ISSP, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
3. Department of Physics, Kyushu University, Fukuoka 812-8581, Japan
4. Department of Chemistry, Kyoto University, Kyoto 615-8510, Japan
5. Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, U.S.A.