Response Of Liquid Water Research Articles

Response of Liquid Water and Vapor Flow to Rainfall Events in Sandy Soil of Arid and Semi-Arid Regions

In arid and semi-arid regions, rainfall takes on a critical significance to both agricultural and engineering construction activities, and the transport process and driving mechanism of soil water under rainfall conditions are in need of further investigation. To clarify the variations in soil moisture, temperature, and liquid and vapor flux under various rainfall scenarios, the Mu Us Sandy Land was selected as the study region, and a water–vapor–heat transport model was established using the Hydrus-1D software with in situ observed soil and meteorological data. The simulated results were in good agreement with the measured data during both the calibration and validation periods, suggesting that the model was accurate and applicable to the study region. The variations in the selected dry and rainy periods proved the significant effect of rainfall events on soil matric potential, temperature, and driving forces. When rainfall occurred, the hydraulic conductivity for liquid water rose by three to five orders of magnitude, driving the liquid water flow downward. In contrast, the vapor flux played a vital role in soil water movement, accounting for about 15% of the total water flux in the shallow layer when the soil was dry, while it became non-significant during rainy periods due to the reduction in hydraulic conductivity for vapor and the temperature gradient. These results clarified the mechanisms of soil liquid water and vapor movement in arid areas, which could provide scientific support for future studies on vegetation restoration and ecosystem sustainability in ecologically fragile areas.

Impact of nuclear quantum effects on the structural inhomogeneity of liquid water

The 2D Raman-terahertz (THz) response of liquid water is studied in dependence of temperature and isotope substitution ([Formula: see text]O, [Formula: see text]O, and [Formula: see text]O). In either case, a very short-lived (i.e., between 75 and 95 fs) echo is observed that reports on the inhomogeneity of the low-frequency intermolecular modes and hence, on the heterogeneity of the hydrogen bond networks of water. The echo lifetime slows down by about 20% when cooling the liquid from room temperature to the freezing point. Furthermore, the echo lifetime of [Formula: see text]O is [Formula: see text] slower than that of [Formula: see text]O, and both can be mapped on each other by introducing an effective temperature shift of [Formula: see text] K. In contrast, the temperature-dependent echo lifetimes of [Formula: see text]O and [Formula: see text]O are the same within error. [Formula: see text]O and [Formula: see text]O have identical masses, yet [Formula: see text]O is much closer to [Formula: see text]O in terms of nuclear quantum effects. It is, therefore, concluded that the echo is a measure of the structural inhomogeneity of liquid water induced by nuclear quantum effects.

2D-Raman-THz spectroscopy: a sensitive test of polarizable water models.

In a recent paper, the experimental 2D-Raman-THz response of liquid water at ambient conditions has been presented [J. Savolainen, S. Ahmed, and P. Hamm, Proc. Natl. Acad. Sci. U. S. A. 110, 20402 (2013)]. Here, all-atom molecular dynamics simulations are performed with the goal to reproduce the experimental results. To that end, the molecular response functions are calculated in a first step, and are then convoluted with the laser pulses in order to enable a direct comparison with the experimental results. The molecular dynamics simulation are performed with several different water models: TIP4P/2005, SWM4-NDP, and TL4P. As polarizability is essential to describe the 2D-Raman-THz response, the TIP4P/2005 water molecules are amended with either an isotropic or a anisotropic polarizability a posteriori after the molecular dynamics simulation. In contrast, SWM4-NDP and TL4P are intrinsically polarizable, and hence the 2D-Raman-THz response can be calculated in a self-consistent way, using the same force field as during the molecular dynamics simulation. It is found that the 2D-Raman-THz response depends extremely sensitively on details of the water model, and in particular on details of the description of polarizability. Despite the limited time resolution of the experiment, it could easily distinguish between various water models. Albeit not perfect, the overall best agreement with the experimental data is obtained for the TL4P water model.

Note: Inverted time-ordering in two-dimensional-Raman-terahertz spectroscopy of water

In a previous paper [P. Hamm and J. Savolainen, J. Chem. Phys. 136, 094516 (2012)]10.1063/1.3691601, we have studied the 2D-Raman-THz response of liquid water, based on an all-atom molecular dynamics simulation. In the pulse sequence we investigated a near-infraredlaser pulse excites a vibrational coherence through a Raman process that switches it into another coherence by a THz pulse after some time, and finally read out by the emission of a THz field.In the present note, we explore the inverted time-ordered pulse sequence, in which the first coherence is generated by a direct THz excitation, while the switching of coherences is achieved by a Raman interaction. Since the dependence of the polarizibility on the nuclear coordinates is more nonlinear compared to that of the dipole moment, this pulse sequence reveals more of the couplings and correlations between the various degrees of freedom of water. Compared to 2D-Raman spectroscopy and the original pulse sequence, this pulse sequence reveals the clearest signature of a photon echo.

Two-dimensional-Raman-terahertz spectroscopy of water: Theory

We discuss the hybrid 2D-Raman-THz spectroscopy of liquid water. This two-dimensional spectroscopy is designed to directly work in the low-frequency range of the intermolecular degrees of freedom. The information content of 2D-Raman-THz spectroscopy is similar to 2D-Raman or 2D-THz spectroscopy, but its experimental implementation should be easier. That is, 2D-Raman-THz spectroscopy is a 3rd-order nonlinear spectroscopy and as such completely avoids cascading of consecutive 3rd-order signals, which turned out to be a major difficulty in 5th-order 2D-Raman spectroscopy. On the other hand, it does not require any intense THz pump-pulse, the lack of which limits 2D-THz spectroscopy to the study of semiconductor quantum wells as the currently available pulse energies are too low for molecular systems. In close analogy to 2D-Raman spectroscopy, the 2D-Raman-THz response of liquid water is simulated from an all-atom molecular dynamics simulation, and the expected spectral features are discussed.

Dynamic response of liquid water to an external static electric field at T=250 K

Dynamic response of liquid water to an external static electric field at T=250 K