Ac technique for simultaneous study of local and global linear responses near the glass transition: The case of doped Ca²⁺/K⁺/NO₃^-

We describe what we believe may be the first linear response experiment which simultaneously measures global and local relaxation kinetics in a viscous liquid near its glass transition temperature. The system chosen for study is the model Ca²⁺/K⁺/NO₃^- ionic liquid {2Ca(NO₃)₂·3KNO₃, or CKN}. Using a low-amplitude temperature-cycling perturbation, we measure the in-phase and out-of-phase components of both the specific heat and the d-d electronic absorption spectrum due to the probe ion Co²⁺. Due to a temperature-dependent first coordination shell exchange, the spectrum oscillates in phase with temperature except where the exchange time scale overlaps the temperature cycle time. From the complex response kinetics, we find that this local exchange process occurs on a shorter time scale and with a narrower relaxation spectrum than does the global response, in accord with hierarchical relaxation concepts.