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Issue 41, 2018
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Reconsideration of the relaxational and vibrational line shapes of liquid water based on ultrabroadband dielectric spectroscopy

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Abstract

The Debye relaxation function is widely used to describe the large dielectric dispersion of ambient water around 20 GHz. However, from a theoretical point of view, this function is supposed to give incorrect predictions at high frequencies owing to the inappropriate assumption that inertial effects and intermolecular interactions do not affect the relaxation dynamics. Our ultrabroadband spectroscopy investigation of liquid water ranging from 500 MHz to 400 THz did demonstrate that the Debye function is inaccurate far above the microwave region. As an alternative, we tried a stochastic frequency modulation (SFM) model assuming instantaneous modification of the line shapes by the correlation with the surrounding system. The SFM relaxation model reproduced the experimental dielectric spectra up to 400 THz, showing that the hydrogen-bond dynamics are associated with the inertial effect that causes the non-exponential relaxation behaviour in a very short time (typically 25 fs). Within the framework of this relaxation model, the hindered translation modes are able to be approximated as fast modulation (homogeneous) line shapes because the interaction time with frequency modulation is too short. Compared with them, the libation mode is found to have a relatively slow modulation (inhomogeneous) origin, where disturbance of water hydrogen bonds induced by the hindered translations leads to fluctuations in the libration frequency.

Graphical abstract: Reconsideration of the relaxational and vibrational line shapes of liquid water based on ultrabroadband dielectric spectroscopy

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Article information


Submitted
27 Jul 2018
Accepted
06 Oct 2018
First published
08 Oct 2018

Phys. Chem. Chem. Phys., 2018,20, 26200-26209
Article type
Paper
Author version available

Reconsideration of the relaxational and vibrational line shapes of liquid water based on ultrabroadband dielectric spectroscopy

K. Shiraga, K. Tanaka, T. Arikawa, S. Saito and Y. Ogawa, Phys. Chem. Chem. Phys., 2018, 20, 26200
DOI: 10.1039/C8CP04778B

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