Fast rotational reorientation of ${\mathrm{NO}}_3^{\mathrm{-}}$ ions and its relation to the glass transition in ${\mathrm{Ca}}_{0.4}{\mathrm{K}}_{0.6}{(\mathrm{N}\mathrm{O}}_3{)}_{1.4}$ liquid: A ${}^{15}\mathrm{N}$ NMR spin-lattice-relaxation study

The fast reorientational dynamics of the ${\mathrm{NO}}_3^{\mathrm{-}}$ ions in the supercooled ${\mathrm{Ca}}_{0.4}{\mathrm{K}}_{0.6}{(\mathrm{N}\mathrm{O}}_3{)}_{1.4}$ (CKN) liquid have been investigated in the temperature range $370\mathrm{K}<~T<~440\mathrm{K}$ by ${}^{15}\mathrm{N}$ NMR spin-lattice-relaxation spectroscopy. The spin-lattice relaxation of the ${}^{15}\mathrm{N}$ nuclides is shown to be controlled by the spin rotation process. The correlation time ${\tau }_c$ of the rapid reorientational dynamics of the ${\mathrm{NO}}_3^{\mathrm{-}}$ ions associated with spin rotation is found to be strongly decoupled from the shear/structural relaxation time scale in this temperature range, varying from ${10}^{-11.8}\mathrm{s}$ at 370 K to ${10}^{-12.5}\mathrm{s}$ at 440 K. An anomalous bifurcation in ${\tau }_c$ into a slow and a fast branch is observed at $T<~365\mathrm{K}$ and is found to be intimately related to the appearance of a bimodal spatial heterogeneity in the overall reorientation or tumbling dynamics of the ${\mathrm{NO}}_3^{\mathrm{-}}$ ions in the supercooled CKN liquid near glass transition. In spite of the large decoupling in the time scale, the fast rotational reorientation is shown to be intimately related to the slow tumbling dynamics, resulting in a two-step decay of the orientational correlation of the ${\mathrm{NO}}_3^{\mathrm{-}}$ ions. The results are found to be compatible with the predictions of the mode coupling theory and offer significant physical insight into the hierarchical nature of the dynamics associated with glass transition.