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.