1. Graduate School of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan
2. Department of Physics, Chiba University, Yayoi-cho 1-33,
Inage-ku, Chiba 263-8522, Japan
3. Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Warsaw 01-224, Poland