1. Department of Chemistry and Biotechnology, The University of Tokyo, 7‐3‐1 Hongo, Bunkyo‐ku, Tokyo 113, Japan
2. Department of Specialty Materials, Chiba University, 1‐33 Yayoi‐cho, Inage‐ku, Chiba 263, Japan
3. Asahi Chemical Industry Company, Limited, Fuji, Japan
4. Japan Atomic Energy Research Institute, Takasaki Radiation Chemistry Research Establishment, 1233 Watanuki‐machi, Takasaki, Gunma 370‐12, Japan