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Issue 28, 2017
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The origin of the Debye relaxation in liquid water and fitting the high frequency excess response

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Abstract

We critically review the literature on the Debye absorption peak of liquid water and the excess response found on the high frequency side of the Debye peak. We find a lack of agreement on the microscopic phenomena underlying both of these features. To better understand the molecular origin of Debye peak we ran large scale molecular dynamics simulations and performed several different distance-dependent decompositions of the low frequency dielectric spectra, finding that it involves processes that take place on scales of 1.5–2.0 nm. We also calculated the k-dependence of the Debye relaxation, finding it to be highly dispersive. These findings are inconsistent with models that relate Debye relaxation to local processes such as the rotation/translation of molecules after H-bond breaking. We introduce the spectrumfitter Python package for fitting dielectric spectra and analyze different ways of fitting the high frequency excess, such as including one or two additional Debye peaks. We propose using the generalized Lydanne–Sachs–Teller (gLST) equation as a way of testing the physicality of model dielectric functions. Our attempts at fitting the experimental spectrum using the gLST relation as a constraint indicate that the traditional way of fitting the excess response with secondary and tertiary Debye relaxations is problematic. All of our work is consistent with the recent theory of Popov et al. (2016) that Debye relaxation is due to the migration of Bjerrum-like defects in the hydrogen bond network. Under this theory, the mechanism of Debye relaxation in liquid water is similar to the mechanism in ice, but the heterogeneity and power-law dynamics of the H-bond network in water results in excess response on the high frequency side of the peak.

Graphical abstract: The origin of the Debye relaxation in liquid water and fitting the high frequency excess response

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Publication details

The article was received on 03 May 2017, accepted on 03 Jul 2017 and first published on 05 Jul 2017


Article type: Paper
DOI: 10.1039/C7CP02884A
Citation: Phys. Chem. Chem. Phys., 2017,19, 18739-18749
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    The origin of the Debye relaxation in liquid water and fitting the high frequency excess response

    D. C. Elton, Phys. Chem. Chem. Phys., 2017, 19, 18739
    DOI: 10.1039/C7CP02884A

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