Dielectric study of normal alcohols with THz time-domain spectroscopy

Complex permittivity measurements of a series of normal alcohols (methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol) are performed by THz time-domain spectroscopy in the frequency range of 0.2-2.5 THz at temperatures of 253-323 K. It is found that the complex permittivity includes the following three components: (i) the high-frequency tail of the dielectric relaxation process, (ii) a mode around 1.2 THz that exhibits the features of a broad vibration mode, and (iii) the low-frequency tail of an intermolecular stretching mode located above 2.5 THz. At low temperatures, the relaxation process moves to a low-frequency range and a peak of the broad vibration mode, which is independent of temperature, is clearly observed around 1.2 THz. This mode is attributed to the fluctuation of the alkyl groups within the hydrogen-bonded chain-like network. Shapes of all the dielectric spectra in the abovementioned THz range indicate that high-frequency dynamics, which emerge in this frequency range, have the same molecular origin in all the samples studied.