Pseudo-Optical Modes in Room-Temperature Ionic Liquids.

The terahertz spectrum encodes information about dynamics, structure, and intermolecular interactions of liquids being probed both experimentally and computationally by techniques such as Raman and far-infrared spectroscopies and molecular dynamics (MD) simulation. In the case of room temperature ionic liquids (RTILs), there has been a debate whether a mode observed at about 1.5-2.7 THz (50-90 cm-1) is due to a quasi-lattice structure or the formation of complex ions. Here we show through the analysis of Raman and far-infrared spectra and MD simulation of a typical RTIL that this mode has a collective optic-like character. Then, employing a simple model based on the theory for optical phonons in crystals, we show that a correlation between the frequency of this mode and material parameters holds for different RTILs. These results, which encompass a wide range of samples, reinforce a quasi-lattice view of the liquid phase.