1. Contribution from the Dipartimento di Chimica, Università di Perugia, 06123 Perugia, Italy, Department of Applied Physics, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8551, Japan, and Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan