1. Department of Applied Physics, Tokyo Institute of Technology, 2-12-1 Oh-okayama, Meguro-ku, Tokyo 152-8551, and Kanagawa Academy of Science and Technology (KAST), KSP Building, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan
2. Department of Applied Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
3. Institute of Physical and Theoretical Chemistry, Technical University of Wroclaw, Wybrzeze, Wyspianskiego 27, 50-370, Wroclaw, Poland