Spectrum Of Liquid Water Research Articles

Temperature dependence analysis of the NIR spectra of liquid water confirms the existence of two phases, one of which is in a coherent state

BackgroundIsosbestic (equal absorption) points in the IR and NIR spectra of liquid water are a well-known feature and they witness the existence of two populations of oscillators in the probed system. Despite it is a well-known experimental fact, in the mainstream molecular dynamics approach the proposed theoretical explanations for it are not able to elucidate which is the physical reason why such a “cut-off frequency” (at the isosbestic point) does exist. MethodsWe investigate pure Milli-Q water on increasing the temperature in the vis-NIR range (400–2500 nm). We specifically payed attention to the first overtone region (1300–1600 nm) of the OH-bond stretching-mode where an isosbestic point has been observed. ResultsA second derivative analysis clearly shows two modes, which can be assigned to water molecules involved in different “hydrogen bonding” configurations whose relative abundance is controlled by the temperature. We have also observed that the ratio of these modes follows a van't Hoff behavior supporting that their energy difference (energy gap) is independent on the temperature. Furthermore, a log-log plot shows a scale invariance of the population ratio with respect to the perturbation (temperature), confirming the existence of a long-range correlated dynamics in the liquid. ConclusionsWe show that the two phases differences between energy and entropy estimated from the experimental data can be compared with the prediction of Quantum Electro Dynamics (QED) showing a remarkable agreement.

Measurements of ultrafast dissociation in resonant inelastic x-ray scattering of water.

There has been a discussion on the interpretation of the resonant inelastic x-ray scattering (RIXS) spectra of liquid water in terms of either different structural environments or that core hole dynamics can generate well-resolved dissociative spectral components. We have used RIXS with high resolution in the OH stretch vibration energy part, at extremely high overtones going toward the continuum of full OH bond breakage, to identify the amount of dissociative contributions in the valence band RIXS spectra at different excitation energies. We observe that at low excitation energies, corresponding to population of states with strongly antibonding character, the valence band RIXS spectra have a large contribution from a well-resolved dissociative feature. Instead, at higher excitations, this spectral component diminishes and becomes a weak structure on the high-energy side of one of the spectral peaks related to the 1b1 state from tetrahedral configurations. This result brings both interpretations to be essential for the understanding of RIXS spectra of liquid water.

Optical properties of ice and snow.

The interactions of electromagnetic radiation with ice, and with ice-containing media such as snow and clouds, are determined by the refractive index and absorption coefficient (the 'optical constants') of pure ice as functions of wavelength. Bulk reflectance, absorptance and transmittance are further influenced by grain size (for snow), bubbles (for glacier ice and lake ice) and brine inclusions (for sea ice). Radiative transfer models for clouds can also be applied to snow; the important differences in their radiative properties are that clouds are optically thinner and contain smaller ice crystals than snow. Absorption of visible and near-ultraviolet radiation by ice is so weak that absorption of sunlight at these wavelengths in natural snow is dominated by trace amounts of light-absorbing impurities such as dust and soot. In the thermal infrared, ice is moderately absorptive, so snow is nearly a blackbody, with emissivity 98-99%. The absorption spectrum of liquid water resembles that of ice from the ultraviolet to the mid-infrared. At longer wavelengths they diverge, so microwave emission can be used to detect snowmelt on ice sheets, and to discriminate between sea ice and open water, by remote sensing. Snow and ice are transparent to radio waves, so radar can be used to infer ice-sheet thickness. This article is part of the theme issue 'The physics and chemistry of ice: scaffolding across scales, from the viability of life to the formation of planets'.

Beyond Badger's Rule: The Origins and Generality of the Structure-Spectra Relationship of Aqueous Hydrogen Bonds.

The structure of hydrogen bonded networks is intimately intertwined with their dynamics. Despite the incredibly wide range of hydrogen bond strengths encountered in water clusters, ion-water clusters, and liquid water, we demonstrate that the previously reported correlation between the change in the equilibrium bond length of the hydrogen bonded OH covalent bond and the corresponding shift in its harmonic frequency in water clusters is much more broadly applicable. Surprisingly, this correlation describes the ratios for both the equilibrium OH bond length/harmonic frequency and the vibrationally averaged bond length/anharmonic frequency in water, hydronium water, and halide water clusters. Consideration of harmonic and anaharmonic data leads to a correlation of -19 ± 1 cm-1/0.001 Å. The fundamental nature of this correlation is further confirmed through the analysis of ab initio Molecular Dynamics (AIMD) trajectories for liquid water. We demonstrate that this simple correlation for both harmonic and anharmonic systems can be modeled by the response of an OH bond to an external field. Treating the OH bond as a Morse oscillator, we develop analytic expressions, which relate the ratio of the shift in the vibrational frequency of the hydrogen-bonded OH bond to the shift in OH bond length, to parameters in the Morse potential and the ratio of the first and second derivatives of the field-dependent projection of the dipole moment of water onto the hydrogen-bonded OH bond. Based on our analysis, we develop a protocol for reconstructing the AIMD spectra of liquid water from the sampled distribution of the OH bond lengths. Our findings elucidate the origins of the relationship between the molecular structure of the fleeting hydrogen-bonded network and the ensuing dynamics, which can be probed by vibrational spectroscopy.

Turbulence Structure in a Stratocumulus Cloud

The growth of computing power combined with advances in modeling methods can yield high-fidelity simulations establishing numerical simulation as a key tool for discovery in the atmospheric sciences. A fine-scale large-eddy simulation (LES) utilizing 1.25 m grid resolution and 5.12 × 5.12 km 2 horizontal domain is used to investigate the turbulence and liquid water structure in a stratocumulus cloud. The simulations capture the observed cloud morphology, including elongated regions of low liquid water path, cloud holes, and pockets of clear air within the cloud. The cloud can be partitioned into two broad layers with respect to the maximum mean liquid. The lower layer resembles convective turbulent structure with classical inertial range scaling of the velocity and scalar energy spectra. The top and shallower layer is directly influenced by the cloud top radiative cooling and the entrainment process. Near the cloud top, the liquid water spectra become shallower and transition to a k − 1 power law for scales smaller than about 1 km .

X-ray spectroscopy with variable line spacing based on reflection zone plate optics.

X-ray spectroscopy is a method, ideally suited for investigating the electronic structure of matter, which has been enabled by the rapid developments in light sources and instruments. The x-ray fluorescence lines of life-relevant elements such as carbon, nitrogen, and oxygen are located in the soft x-ray regime and call for suitable spectrometer devices. In this Letter, we present a high-resolution spectrum of liquid water, recorded with a soft x-ray spectrometer based on a reflection zone plate (RZP) design. The RZP-based spectrometer with meridional variation of line space density from 2953 to 3757l/mm offers extremely high detection efficiency and, at the same time, medium energy resolution. We can reproduce the well-known splitting of liquid water in the lone pair regime with 10s acquisition time.

Fermi resonance in OH-stretch vibrational spectroscopy of liquid water and the water hexamer.

Vibrational spectroscopy of water contains a wealth of information about the structure and dynamics of this fascinating substance. Theoretical modeling of fundamental vibrational transitions in condensed water has proven difficult, and in many circumstances, one cannot reach even qualitative agreement with experiment. Due to the ability of water to form hydrogen bonds of various strengths, the OH stretching band spans several hundreds of wave numbers in the spectra, overlapping with the first overtone of the HOH bending band and triggering a resonance between these two vibrations. This effect, known as Fermi resonance, has been traditionally ignored in theoretical condensed-phase simulations due to the additional computational burden and its deemed low importance. Depending on a particular molecular environment, the Fermi resonance manifests itself from small spectral features in the spectra of liquid water to pronounced distinct peaks in the spectra of ice and water clusters. The goal of this work is to illustrate the effects of including the Fermi resonance coupling between the bending overtone and stretching fundamental vibrations in the mixed quantum-classical formalism developed by Skinner and co-workers on the IR and Raman spectra of liquid water and the water hexamer. We show that by adding the Fermi resonance coupling, we are able to reproduce the location of the peak and a shoulder on the red side of the IR spectrum as well as the bimodal structure of the polarized Raman spectrum of liquid water at 300 K. Very good agreement between theory and experiment is achieved for the IR spectra of the water hexamer as well. We suggest that the Fermi resonance should not be ignored if intricate features of spectra are of interest. In spite of these promising results obtained in the region of a spectrum where Fermi resonance is important, further development of spectroscopic maps is needed to improve agreement with the experiment outside of the frequency range affected by the Fermi resonance.

Absolute Energy Levels of Liquid Water.

We determine the ionization potential (IP) and the electron affinity (EA) of liquid water together with the absolute redox level of the standard hydrogen electrode (SHE) by combining advanced electronic-structure calculations, ab initio molecular dynamics simulations, thermodynamic integration, and potential alignment at the water-vacuum interface. The calculated SHE level lies at 4.56 eV below the vacuum level, close to the experimental reference of 4.44 eV inferred by Trasatti. The band edges are determined through a hybrid functional designed to reproduce the band gap achieved with highly accurate GW calculations. Our analysis yields IP = 9.7 eV and EA = 0.8 eV, consistent with both photoemission spectra of liquid water and thermodynamical data for the hydrated electron.

Laser based measurement of water film thickness for the application in exhaust after-treatment processes

In this work, an absorption based laser sensor for the investigation of dynamically changing liquid film thicknesses is developed and validated. For the wavelength selection of the single-ended, fibre-coupled, diode-laser sensor, near-infrared spectra of liquid water are measured at various thicknesses and temperatures. To reduce the influence of the film temperature, the evaluation is supported by a calibration procedure. Unknown film thicknesses of up to 440 µm could then be measured. To adapt the sensor to particular boundary conditions of different systems, the single-ended setup can be changed to a transmissive configuration. The sensor is validated in both configurations against a commercially available chromatic confocal resonance (CHR) sensor. A comparison with respect to the CHR sensor leads to an accuracy of 2.8 µm and a precision of 3.9% of the new laser based sensor.

Transitions energies, optical oscillator strengths and partial potential energy surfaces of inner-shell states of water clusters

Transitions energies, optical oscillator strengths and partial potential energy surfaces of inner-shell (core–hole) states of water clusters are calculated at the inner-shell multiconfigurational self-consistent field (IS-MCSCF) level. This approach has been applied to diatomic and small polyatomic molecules with high accuracy. In the present work, IS-MCSCF was applied to molecular clusters for the first time. Discussions are carried out based on calculations done at different geometric conformations. It is shown that the transition energy is blue shifted to the first two discrete states at O 1s edge when referred to the isolated water molecule, while the core-electron binding energy is red shifted. Although the results for water clusters can only be compared to liquid water with care, it is worth emphasizing that this same trend is found in the corresponding spectra of the latter, revealing the importance of the first solvation shell to establish the main features of liquid water spectrum.

The Interplay of Structure and Dynamics in the Raman Spectrum of Liquid Water over the Full Frequency and Temperature Range.

While many vibrational Raman spectroscopy studies of liquid water have investigated the temperature dependence of the high-frequency O-H stretching region, few have analyzed the changes in the Raman spectrum as a function of temperature over the entire spectral range. Here, we obtain the Raman spectra of water from its melting to boiling point, both experimentally and from simulations using an ab initio-trained machine learning potential. We use these to assign the Raman bands and show that the entire spectrum can be well described as a combination of two temperature-independent spectra. We then assess which spectral regions exhibit strong dependence on the local tetrahedral order in the liquid. Further, this work demonstrates that changes in this structural parameter can be used to elucidate the temperature dependence of the Raman spectrum of liquid water and provides a guide to the Raman features that signal water ordering in more complex aqueous systems.

Raman spectroscopic and theoretical study of liquid and solid water within the spectral region 1600–2300 cm−1

Raman spectra of liquid water and ice were measured at different temperatures. The intensity of the band assigned to bending vibrations of water molecules was observed to decrease at the liquid-to-solid transition, while the Raman line near 2200cm−1 showed an anomalously high intensity in the solid phase. A tetrahedral model was used for computer analysis of the observed spectral changes. Quantum-chemical calculations of the structure, normal vibrations and Raman spectra in the harmonic approximation, as well as frequencies and intensities of some vibrations using 1D and 2D potential energy surfaces, were carried out using B3LYP with the cc-pVTZ basis set. The influence of the number of hydrogen bonds on the frequency and Raman activity of the bending vibrations was analyzed. The possibility of hydrogen bond weakening upon excitation of the combined bending-rocking vibration due to the large amplitude of this vibration is considered.

Absorption spectrum of water as a reflection of charge diffusion

The absorption spectrum of liquid water in the range of 103–1013 Hz (from dc conductivity of up to 5 THz IR resonance) is completely and uniformly described by a simple physical model. The model assumes the existence in water of a high concentration of H3O+ and OH− ions (up to 4.5% of the total number of molecules) that exchange protons with H2O molecules on the scale of picoseconds.

Dynamical behavior of molecular partial charges implied by the far-infrared spectral profile of liquid water

On the basis of the successful reproducibility of the band at ∼200 cm−1 (6 THz) in the far-infrared (terahertz) spectrum of liquid water by spectral simulations, and the intermolecular charge flux as the primary mechanism giving rise to the intensity of this band, the dynamical behavior of the molecular partial charges in liquid water implied by the presence of this band is examined theoretically. The molecular partial charges are typically within the range of ±0.03 e, with frequent short-time (<0.1 ps) excursions out of this range, and are rapidly modulated with a time constant of ∼0.12 ps. The long-time behavior of the time correlation function in the 0.3–10 ps region is related to the topological nature of the related molecular motions. The implication of the results on the use of time-domain terahertz spectroscopy to detect the properties of molecular motions in various environments is discussed.

Possible mechanism of molecular motion in liquid water from dielectric spectroscopy data

The broadband absorption spectrum of liquid water (103–1013Hz) contains a variety of dispersion features spread far apart on the time scale – the dc conductivity, the Debye relaxation, the relaxation-2, and the intermolecular vibration absorption peak at 5.3 THz. In the framework of the common hydrogen-bonding paradigm, the unified description of the features is not available. In an attempt to build a unified picture, we utilize the Frenkel's idea of the oscillation-translation dynamic of particles as a fundamental type of motion inherent to liquids. We develop a model of water structure, in which the dynamics is due to diffusion of particles, neutral H2O molecules and H3O+ and OH– ions - with their periodic localizations and mutual transformations. This model establishes, for the first time, a consistent link between the quoted spectral phenomena. Such currently debatable issues, as anomalous proton mobility and autoionization of water are discussed in the light of the new model. According to the model, water possesses a huge amount of short-living counter ions. This strongly contradicts to many current views on water, but meets support in individual MD simulations.

Cooperation and Environment Characterize the Low-Lying Optical Spectrum of Liquid Water.

The optical spectrum of liquid water is analyzed by subsystem time-dependent density functional theory. We provide simple explanations for several important (and so far elusive) features. Due to the disordered environment surrounding each water molecule, the joint density of states of the liquid is much broader than that of the vapor, thus explaining the red-shifted Urbach tail of the liquid compared to the gas phase. Confinement effects provided by the first solvation shell are responsible for the blue shift of the first absorption peak compared to the vapor. In addition, we also characterize many-body excitonic effects. These dramatically affect the spectral weights at low frequencies, contributing to the refractive index by a small but significant amount.

X-ray absorption of liquid water by advanced ab initio methods

Oxygen K-edge X-ray absorption spectra of liquid water are computed based on the configurations from advanced ab initio molecular dynamics simulations, as well as an electron excitation theory from the GW method. One one hand, the molecular structures of liquid water are accurately predicted by including both van der Waals interactions and hybrid functional (PBE0). On the other hand, the dynamic screening effects on electron excitation are approximately described by the recently developed enhanced static Coulomb hole and screened exchange approximation by Kang and Hybertsen [Phys. Rev. B 82, 195108 (2010)]. The resulting spectra of liquid water are in better quantitative agreement with the experimental spectra due to the softened hydrogen bonds and the slightly broadened spectra originating from the better screening model.

Disentangling structural information from core-level excitation spectra.

Core-level spectra of liquids can be difficult to interpret due to the presence of a range of local environments. We present computational methods for investigating core-level spectra based on the idea that both local structural parameters and the x-ray spectra behave as functions of the local atomic configuration around the absorbing site. We identify correlations between structural parameters and spectral intensities in defined regions of interest, using the oxygen K-edge excitation spectrum of liquid water as a test case. Our results show that this kind of analysis can find the main structure-spectral relationships of ice, liquid water, and supercritical water.

Orthogonalyzed H2O and D2O species obtained from infrared spectra of liquid water at several temperatures

Infrared spectra of light and heavy liquid waters were obtained between room temperature and 100 °C. Stretch regions show large structureless bands whose intensities decrease with temperature with slight blue shifts. Isosbestic points indicate two types whose abundances vary with temperature: cold and hot waters. Second derivatives and band simulations were used in synergy to untangle spectral band components. For cold light and hot waters, six Gaussian components make up almost 98% of the 3-μm absorption. In these, the two principal components are ν3 and ν1. The last 2% are made of five weaker components. Likewise, the cold and hot heavy water 4-μm spectra are made of six principal (∼95%) and seven secondary components. Between ν3 and ν1 lie 2ν2 components: strong in heavy water and weak in light water. For both waters, hot species are slightly blue shifted with, however, a 40% intensity decrease. Comparison of the light water cold-hot difference spectrum with that of normalized heavy water indicates a major difference arising from the prominent presence of 2ν2 components in heavy water spectra.

Reliable absorbance measurement of liquid samples in soft X-ray absorption spectroscopy in transmission mode

In soft X-ray absorption spectroscopy in transmission mode (T-mode XAS), it is necessary to optimize the thickness of thin liquid layers for appropriate absorbance around 0.3–0.7 and keep the sample thickness flat within a photon beam. In the present work at the UVSOR Synchrotron Facility, a thin liquid layer is sandwiched between two 100nm thick Si3N4 or SiC membranes of 2×2mm2 in a helium chamber with enough working space, and the liquid thickness is optimized by using the elasticity of the membranes under controlling the helium pressure between 0.1013 and 0.12MPa. The flatness of the liquid sample within a photon beam is investigated with measuring position-dependent O K-edge T-mode XAS spectra of liquid water. In the case of the beam size of 50μm and the helium pressure of 0.1090MPa (0.1038MPa), it is confirmed that the liquid water layer has flat thickness around 200nm (320nm) at the center position of 2×2mm2 sample area with allowance of ±350μm (±150μm), where the thickness is ca 900nm at the brim of the liquid sample area. The pre-edge vs. main edge (Pre/Main) ratio is 0.38±0.01 for the center of the sample area and becomes considerably larger than 0.38 for the off-center position. The deviation from 0.38 is caused by the inhomogeneous thickness and is evaluated in comparison with a model simulation. The Pre/Main ratio can be used to evaluate the flatness in the liquid sample thickness within the beam area of soft X-rays. Positioning of as small a beam as possible on the center of the liquid sample area is essential to obtain reliable T-mode XAS spectra with the present liquid cells.