1. Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240, Japan, Yokohama Research Center R&D Division, Mitsubishi Chemical Corporation, Kamoshida-cho 1000, Aoba-ku, Yokohama 227, Japan, and Department of Application Laboratories, Center for Applied Technology, Rigaku Corporation, Matsubara-cho 3-9-12, Akishima-shi, Tokyo 196, Japan