1. Institute of Environmental Science and Technology, Yokohama National University, Tokiwadai, Hodogaya‐ku, Yokohama 240, Japan
2. Fundamental Research Laboratories, NEC Corporation, Miyamae‐ku, Kawasaki 213, Japan
3. Research Institute of Electrical Communication, Tohoku University, Katahira, Sendai 980, Japan
4. Research Institute, National Rehabilitation Center of Disabled, Namiki, Tokorozawa, Saitama 359, Japan
5. Department of Industrial Chemistry, Science University of Tokyo, Yamazaki, Noda, Chiba 278, Japan