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We have measured the linear and nonlinear dielectric responses of S-methoxypropylene carbonate, a highly polar glass-former, for which it has been reported that the “hump,” which is typical of third harmonic susceptibilities, disappears across a 5 K temperature change. To understand this unusual feature, we have measured the responses to high amplitude ac and dc electric fields at the fundamental frequency. The static limits of these results are entered into a model aimed at reproducing nonlinear dielectric susceptibility spectra using the concept of a fictive electric field. This model reproduces the “hump” in the third-harmonic response and its seeming disappearance. It is revealed that the “hump” is predominantly the result of reduced time constants, a consequence of the energy the sample absorbs from the electric field. At elevated temperatures, the “hump” only appears to vanish because its reduced amplitude submerges below the extraordinarily high level of polarization saturation of this liquid.
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Second harmonic generation observed by dielectric relaxation at high electric fields: SHG without optics
We have measured second harmonic generation via the nonlinear dielectric permittivity of a polar glass-forming liquid, propylene glycol, with the inversion symmetry of the liquid broken by applying a dc bias field. For a given combined peak field, EB + E0, the highest second harmonic signals are obtained when the dc bias field and peak ac-field have the same amplitude, i.e., EB = E0. Second harmonic results measured in the static limit agree well with the theoretical prediction based upon the third-order nonlinear susceptibility term that connects polarization P with the static field E. Second harmonic signals are detectable at all frequencies at which dipole orientation contributes to permittivity, possibly occurring the instant a dc bias field is applied. The results imply that second harmonic generation as a signature of anisotropy, e.g., at interfaces, can be assessed by impedance spectroscopy at twice the fundamental frequency, not only by optical techniques.
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- Award ID(s):
- 2153944
- PAR ID:
- 10661317
- Publisher / Repository:
- AIPP
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 163
- Issue:
- 24
- ISSN:
- 0021-9606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Molecular and collective reorientations in interfacial water are by-and-large decelerated near surfaces subjected to outgoing electric fields (pointing from surface to liquid, i.e., when the surface carries positive charge). In incoming fields at negatively charged surfaces, these rates show a nonmonotonic dependence on field strength where fastest reorientations are observed when the field alignment barely offsets the polarizing effects due to interfacial hydrogen bonding. This extremum coincides with a peak of local static permittivity. We use molecular dynamics simulations to explore the impact of background static field on high frequency AC permittivity in hydration water under an electric field mimicking the conditions inside a capacitor where one of the confinement walls is subject to an outgoing field and the other one to an incoming field. At strong static fields, the absorption peak undergoes a monotonic blue shift upon increasing field strength in both hydration layers. At intermediate fields, however, the hydration region at the wall under an incoming field (the negative capacitor plate) features a red shift coinciding with maximal static-permittivity and reorientation-rate. The shift is mostly determined by the variation of the inverse static dielectric constant as proposed for mono-exponentially decaying polarization correlations. Conversely, hydration water at the opposite (positively charged) surface features a monotonic blue shift consistent with conventional saturation. The sensitivity of absorption peaks on the field suggests that surface charge densities could be deduced from sub-THz dielectric spectroscopy experiments in porous materials when interfaces accommodate a major fraction of water contained in the system.more » « less
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1063/5.0309351
