Abstract
The local structure of liquid water as a function of temperature is a source of intense research. This structure is intimately linked to the dynamics of water molecules, which can be measured using Raman and infrared spectroscopies. The assignment of spectral peaks depends on whether they are collective modes or single-molecule motions. Vibrational modes in liquids are usually considered to be associated to the motions of single molecules or small clusters. Using molecular dynamics simulations, here we find dispersive optical phonon-like modes in the librational and OH-stretching bands. We argue that on subpicosecond time scales these modes propagate through water's hydrogen-bond network over distances of up to 2 nm. In the long wavelength limit these optical modes exhibit longitudinal–transverse splitting, indicating the presence of coherent long-range dipole–dipole interactions, as in ice. Our results indicate the dynamics of liquid water have more similarities to ice than previously thought.
Highlights
The local structure of liquid water as a function of temperature is a source of intense research
We wish to study the k dependence of the dielectric susceptibility, where k 1⁄4 2p/l. k À dependence cannot be probed directly by experiment, but in the limit of infinite wavelength (k-0) the longitudinal and transverse dielectric susceptibilities can be obtained from the dielectric function via the following relations[32,33]: wLðk wT ðk ! 0; oÞ 1⁄4 eðoÞ À 1
In this work, we have presented several lines of evidence for short-lived optical phonons that propagate along the H-bond network of water
Summary
The local structure of liquid water as a function of temperature is a source of intense research. These three peaks were assigned to the three librational motions of the water molecule—twisting (E435 cm À 1), rocking (E600 cm À 1) and wagging (E770 cm À 1)[8,9,11] When comparing these assignments with infrared and dielectric spectra, one runs into a serious discrepancy. It has previously been shown that the librational peak in the longitudinal dielectric susceptibility of water is dispersive[20], and Bopp and Kornyshev noted that the dispersion relation has the appearance of an optical phonon mode[21]. One way to understand LO–TO splitting is through the Lyddane–Sachs–Teller (LST) relation[23]: o2LO o2TO This relation was originally derived for a cubic ionic crystal it was later shown to have very general applicability[24,25], and has been applied to disordered and glassy solids[17,26,27]. The generalized LST relation is[24]:
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