Comparative dielectric study of pentanol isomers with terahertz time-domain spectroscopy
Introduction
“THz waves” is a generic term used to refer to electromagnetic waves of 0.1–10 × 1012 Hz in frequency. Before efficient methods of THz wave emission and detection were established, the THz region was not studied extensively. However, emission and detection techniques for THz waves have been developed with a femtosecond laser and semiconductor device technology in recent years. THz waves are now attracting attention in fundamental and applied research. In the former, THz time-domain spectroscopy (THz-TDS) has been established [1], [2], enabling us to accurately measure complex permittivity in the THz region.
The new experimental technique has proven to be a powerful tool for the study of molecular dynamics and interactions in various materials. The study of molecular dynamics in hydrogen-bonded liquids is one of the most important applications because of their significance in various fields such as physical chemistry, biology, and industry. In the THz region, molecular dynamics are expected to be related to the hydrogen-bonding phenomena that distinguish hydrogen-bonded liquids from simple liquids. Observation will therefore serve as a key to explaining complicated physical properties in hydrogen-bonded liquids.
In the present study, we have performed complex permittivity measurements of pentanol isomers (1-pentanol, 2-pentanol, 3-pentanol, 2-methyl-1-butanol, 3-methyl-1-butanol, 2-methyl-2-butanol, and 3-methyl-2-butanol) with THz-TDS. These structural isomers differ in the position of the OH group and the structure of the carbon chain, which affects the formation of hydrogen bonds in the liquid state. The objective of this study is to perform systematic experiments with pentanol isomers in the THz region. On basis of the experimental results, we will discuss the underlying microscopic mechanisms contributing to dielectric spectra in this frequency range.
Section snippets
Experimental
The experimental technique of THz-TDS has been well developed and described in detail by others [3], [4], [5]. A mode-locked Ti sapphire laser (Tsunami, Spectra Physics), which generates femtosecond pulses with the wavelength of 800 nm, was used for the emission and detection of pulsed THz waves. For the emission, we produced a photoconductive antenna with electrodes, the gap distance of which is 500 μm, fabricated on a semi-insulating gallium arsenide substrate. In the detection of THz pulses,
Complex permittivity in the THz region
Fig. 2(a)–(g) show the complex permittivities of all the pentanol isomers studied. Some dielectric relaxation processes below the GHz frequency region in monohydric alcohols have been reported [10], [11], [12], [13], [14]. These processes contribute to in the THz region, resulting in the rise in dielectric permittivity and dielectric loss below 0.5 THz in Fig. 2(a)–(g). With decreasing temperature, these relaxation processes shift to a lower frequency range,
Conclusions
We have measured the complex permittivity of seven pentanol isomers with THz-TDS. The molecular structures of these isomers change systematically with the position of the OH group and the structure of the carbon chain. The complex permittivities studied contain three contributions: (i) a high-frequency side of dielectric relaxation processes, (ii) a vibration mode around 1.5 THz, and (iii) a low-frequency side of an intermolecular stretching mode located above 2.5 THz. The dielectric spectra in
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