1. Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, Vass. Constantinou 48 Avenue, 116 35 Athens, Greece, Fundamental and Environmental Research Laboratories, SORST-JST, c/o 34 Miyukigaoka, Tsukuba, Ibaraki 305-850, Japan, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-850, Japan, and Meijo University, 1-501 Shiogamaguchi, Tenpaku-ku, Nagoya 468-8502, Japan