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.

中文翻译：

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室温离子液体中的伪光学模式

太赫兹光谱编码有关通过拉曼和远红外光谱学以及分子动力学（MD）模拟等技术在实验和计算上探测到的液体的动力学，结构和分子间相互作用的信息。对于室温离子液体（RTIL），一直存在争论，即是否在约1.5–2.7 THz（50–90 cm ^–1）是由于准晶格结构或复合离子的形成。在这里，我们通过分析拉曼光谱和远红外光谱以及典型RTIL的MD模拟表明，该模式具有类似光学的集体特征。然后，采用基于晶体中的光子理论的简单模型，我们表明该模式的频率与材料参数之间的相关性适用于不同的RTIL。这些结果涵盖了广泛的样品范围，增强了液相的准晶格视图。