1. Department of Chemistry, Graduate School of Engineering Science and Research Center for Materials Science at Extreme Conditions, Osaka University, Toyonaka, Osaka 560, Japan; Institut für Physikalische Chemie, Universität Mainz, D-55099 Mainz, Germany; and Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany