Terahertz time domain spectroscopy (THz-TDS) of polar liquids

The dielectric spectrum of water is measured at four temperatures between 0.5°C and 20°C using Terahertz Time-Domain Spectroscopy with a THz spectrometer designed to cover a range from 15 GHz to 2 THz. The frequency dependent dielectric constant is fit using the reduced polarization formula by Fatuzzo and Mason allowing the microscopic dipole correlation function of water, gamma(t), to be related to the to the macroscopic polarization of water, 3&d4;w .; The water spectra are fit to a sum of Debye relaxations and Lorentzian oscillators to accurately model the data over the range of 15 GHz to 2 THz. The fitting includes the higher frequency known terahertz modes of water that have non-zero tails in the range studied. This allows the high frequency limit of the complex permittivity, 3infinity , to be appropriately fit.; The spectra in this range reveal several new oscillations on the order of one percent of the total complex permittivity. These oscillations are fit by both positive and negative Lorentzians, resulting in the first observed incidence of anti-correlated water. Most of these frequencies show negligible temperature dependence over the range of temperatures studied. These oscillations have been confirmed by molecular dynamics simulations and may indicate long range dipole-dipole interactions. To determine if these features show up in other strongly polar, hydrogen-bonded systems, D2O was studied at 20°C. The D2O residual also shows positive and negative correlated effects at similar frequencies as those found in water.; Finally, the water data shows the emergence of a sharp resonance in the residual at 4°C. This resonance appears at about 21 GHz and quickly disappears with temperature, as additional spectra at 3.5 and 4.5°C illustrate. This feature, which does not appear at higher or lower temperatures, is unique in that it is the only feature of water other than the density anomaly to be tied to a specific temperature. All other features of water show linear relationships from supercooled to supercritical temperatures when temperature dependences are found. This implies that this newly discovered feature of water might be structurally related to the liquid water network.