Temperature-dependent dynamic structure factors for liquid water inferred from inelastic neutron scattering measurements

Abstract

Temperature-dependent dynamic structure factors S(Q, ω) for liquid water have been calculated using a composite model, which is based on the decoupling approximation of the mean square displacement of the water molecules into diffusion and solid-like vibrational parts. The solid-like vibrational part S vib (Q, ω) is calculated with the phonon expansion method established in the framework of the incoherent Gaussian approximation. The diffusion part S diff (Q, ω) relies on the Egelstaff-Schofield translational diffusion model corrected for jump diffusions and rotational diffusions with the Singwi-Sjölander random model and Sears expansion, respectively. Systematics of the model parameters as a function of temperature were deduced from quasi-elastic neutron scattering data analysis reported in the literature and from molecular dynamics (MD) simulations relying on the TIP4P/2005f model. The resulting S(Q, ω) values are confronted by means of Monte Carlo simulations to inelastic neutron scattering data measured with IN4, IN5, and IN6 time-of-flight spectrometers of the Institut Laue-Langevin (ILL) (Grenoble, France). A modest range of temperatures (283-494 K) has been investigated with neutron wavelengths corresponding to incident neutron energies ranging from 0.57 to 67.6 meV. The neutron-weighted multiphonon spectra deduced from the ILL data indicate a slight overestimation by the MD simulations of the frequency shift and broadening of the librational band. The descriptive power of the composite model was suited for improving the comparison to experiments via Bayesian updating of prior model parameters inferred from MD simulations. The reported posterior temperature-dependent densities of state of hydrogen in H 2 O would represent valuable insights for studying the collective coupling interactions in the water molecule between the inter-and intramolecular degrees of freedom.

Water Diffusion Proceeds via a Hydrogen-Bond Jump Exchange Mechanism

“…Experimentally, jump models have been used to analyze water QENS spectra, 9,10,[28][29][30][31] and translational jump time values close to the values obtained here have been shown to be consistent with experimental linewidths. 30 However, QENS spectra probe displacements of water H atoms 28 and, while the latter share the same diffusion coefficient as water O atoms, their motions result from the combination of larger-amplitude jumps when the H atom exchanges H-bond acceptors and smaller-amplitude jumps when the parent O atom experiences an H-bond exchange which does not involve this H. A specific translational jump model for water H atoms combining these two types of exchanges would thus provide a way to model QENS spectra without having to use the traditional but questionable translationrotation decoupling approximation.…”

Temperature Dependence of Non-Condon Effects in Two-Dimensional Vibrational Spectroscopy of Water

“…There have been several efforts to study the effects of temperature on the structure and dynamics of pure and isotopically diluted water. ,− These studies have shown that the hydrogen bonds become weaker and break more frequently on increase of temperature. Both the infrared and Raman spectra of the OH stretch shift toward the blue side on increase of temperature.…”

Dynamical correlations in simple disorder and complex disorder liquids

“…for liquid water at ambient conditions and inelastic neutron scattering at various temperatures [43]. MD results of the coherent dynamic structure factor of liquid water at the mesoscale have been presented by Alvarez et al [44].…”