Orientational dynamics of the ionic organic liquid 1-ethyl-3-methylimidazolium nitrate

Optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments are used to study the orientational dynamics of the ionic organic liquid 1-ethyl-3-methylimidazolium nitrate $({\mathrm{EMIM}}^{+}{\mathrm{NO}}_3^{-})$ over time scales from $\text{∼1\hspace{0.17em}}\mathrm{ps}$ to $\text{∼2\hspace{0.17em}}\mathrm{ns},$ and the temperatures range from 410 to 295 K. The temperatures cover the normal liquid state and the weakly supercooled state. The orientational dynamics exhibit characteristics typical of normal organic glass-forming liquids. The longest time scale portion of the data decays as a single exponential and obeys the Debye–Stokes–Einstein relation. The decay of the OHD-OKE signal begins $\text{(∼1\hspace{0.17em}}\mathrm{ps})$ with a temperature independent power law, $t^{\mathrm{-z}},$ $\text{z=1.02±0.05,}$ the “intermediate power law.” The power law decay is followed by a crossover region, modeled as a second power law, the von Schweidler power law. The longest time scale decay is the exponential α relaxation. The intermediate power law decay has been observed in van der Waals supercooled liquids previously. These are the first such observations on an ionic organic liquid. The observation of the dynamical signatures observed in other liquids demonstrates that the orientational dynamics of ionic organic liquids are fundamentally the same as van der Waals liquids and supports the universality of the intermediate power law decay in the dynamics of complex liquids. Within the mode-coupling theory (MCT) framework, the MCT critical temperature $T_C$ is estimated to be $T_C\text{≅255\hspace{0.17em}}\mathrm{K}.$