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Abstract:
Precise dielectric spectra have been determined at 25 degrees C over the exceptionally broad frequency range of 0.1 <}or= nu/GHz {<or= 3000 for the imidazolium-based room-temperature ionic liquids (RTILs) [bmim][BF4], [bmim][PF6], [bmim][DCA], and [hmim][BF4]. The spectra are dominated by a low-frequency process at approximately 1 GHz with a broad relaxation time distribution of the Cole-Davidson or Cole-Cole type, which is thought to correspond to the rotational diffusion of the dipolar cations. In addition, these RTILs possess two Debye relaxations at approximately 5 GHz and approximately 0.6 THz and a damped harmonic oscillation at approximately 2.5 THz. The two higher-frequency modes are almost certainly due to cation librations, but the origin of the approximately 5 GHz mode remains obscure.
