Raman Spectra Of Liquid Water Research Articles

Tensorial Properties via the Neuroevolution Potential Framework: Fast Simulation of Infrared and Raman Spectra.

Infrared and Raman spectroscopy are widely used for the characterization of gases, liquids, and solids, as the spectra contain a wealth of information concerning, in particular, the dynamics of these systems. Atomic scale simulations can be used to predict such spectra but are often severely limited due to high computational cost or the need for strong approximations that limit the application range and reliability. Here, we introduce a machine learning (ML) accelerated approach that addresses these shortcomings and provides a significant performance boost in terms of data and computational efficiency compared with earlier ML schemes. To this end, we generalize the neuroevolution potential approach to enable the prediction of rank one and two tensors to obtain the tensorial neuroevolution potential (TNEP) scheme. We apply the resulting framework to construct models for the dipole moment, polarizability, and susceptibility of molecules, liquids, and solids and show that our approach compares favorably with several ML models from the literature with respect to accuracy and computational efficiency. Finally, we demonstrate the application of the TNEP approach to the prediction of infrared and Raman spectra of liquid water, a molecule (PTAF-), and a prototypical perovskite with strong anharmonicity (BaZrO3). The TNEP approach is implemented in the free and open source software package gpumd, which makes this methodology readily available to the scientific community.

Raman Spectra of Electrified Si-Water Interfaces: First-Principles Simulations.

We investigate the Raman spectra of liquid water in contact with a semiconductor surface using first-principles molecular dynamics simulations. We focus on a hydrogenated silicon-water interface and compute the Raman spectra from time correlation functions of the polarizability. We establish a relationship between Raman spectral signatures and structural properties of the liquid at the interface, and we identify the vibrational impacts of an applied electric field. We show that negative bias leads to a reduction of the number of hydrogen bonds (HBs) formed between the surface and the topmost water layer and an enhancement of the HB interactions between water molecules. Instead, positive bias leads to an enhancement of both the HB interactions between water and the surface and between water molecules, creating a semi-ordered interfacial layer. Our work provides molecular-level insights into electrified semiconductor/water interfaces and the identification of specific structural features through Raman spectroscopy.

Predicting the Raman Spectra of Liquid Water with a Monomer-Field Model.

The Raman spectrum of liquid water is quite complex, reflecting its strong sensitivity to the local environment of the individual waters. The OH-stretch region of the spectrum, which captures the influence of hydrogen bonding, has only just begun to be unraveled. Here we develop a model for predicting the Raman spectra of the OH-stretch region by considering how local electric fields distort the energy surface of each water monomer. We find that our model is capable of reproducing the bimodal nature of the main peak, with the shoulder at 3250 cm-1 resulting almost entirely from Fermi resonance. Furthermore, we capture the temperature and polarization dependence of the shoulder, which has proven to be difficult to obtain with previous methods, and analyze the origin of this dependence. We expect our model to be generally useful for understanding and predicting how Raman spectra change under different conditions and with different probe reporters beyond water.

A Simple Calibration Method of Anti-Stokes-Stokes Raman Intensity Ratios Using the Water Spectrum for Intracellular Temperature Measurements.

Presented here is a facile and practical method for calibrating anti-Stokes-Stokes intensity ratios in low-frequency Raman spectra that is devised specifically for temperature measurements inside cells. The proposed method uses as an intensity standard the low-frequency Raman spectrum of liquid water, a major molecular component of cells, whose temperature is independently measured with a thermocouple. Rather than calibrating pixel intensities themselves, we obtain a correction factor at each Raman shift in the 20-200 cm-1 region by dividing the anti-Stokes-Stokes intensity ratio calculated theoretically from the Boltzmann factor at the known temperature by that obtained experimentally. The validity of the correction curve so obtained is confirmed by measuring water at other temperatures. The anti-Stokes-Stokes intensity ratios that have been subjected to our calibration are well fitted with the Boltzmann factor within ∼1% errors and yield water temperatures in fairly good agreement with the thermocouple temperature (an average difference ∼1 ℃). The present method requires only 15 min of spectral acquisition time for calibration, which is 50 times shorter than that in a recently reported calibration method using the pure rotational Raman spectrum of N2. We envision that it will be an effective asset in Raman thermometry and its applications to cellular thermogenesis and thermoregulation.

Raman spectrum and polarizability of liquid water from deep neural networks.

We introduce a scheme based on machine learning and deep neural networks to model the environmental dependence of the electronic polarizability in insulating materials. Application to liquid water shows that training the network with a relatively small number of molecular configurations is sufficient to predict the polarizability of arbitrary liquid configurations in close agreement with ab initio density functional theory calculations. In combination with a neural network representation of the interatomic potential energy surface, the scheme allows us to calculate the Raman spectra along 2-nanosecond classical trajectories at different temperatures for H2O and D2O. The vast gains in efficiency provided by the machine learning approach enable longer trajectories and larger system sizes relative to ab initio methods, reducing the statistical error and improving the resolution of the low-frequency Raman spectra. Decomposing the spectra into intramolecular and intermolecular contributions elucidates the mechanisms behind the temperature dependence of the low-frequency and stretch modes.

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.

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.

Quantum Dynamics and Spectroscopy of Ab Initio Liquid Water: TheInterplay of Nuclear and Electronic Quantum Effects.

Understanding the reactivity and spectroscopy of aqueous solutions at the atomistic level is crucial for the elucidation and design of chemical processes. However, the simulation of these systems requires addressing the formidable challenges of treating the quantum nature of both the electrons and nuclei. Exploiting our recently developed methods that provide acceleration by up to 2 orders of magnitude, we combine path integral simulations with on-the-fly evaluation of the electronic structure at the hybrid density functional theory level to capture the interplay between nuclear quantum effects and the electronic surface. Here we show that this combination provides accurate structure and dynamics, including the full infrared and Raman spectra of liquid water. This allows us to demonstrate and explain the failings of lower-level density functionals for dynamics and vibrational spectroscopy when the nuclei are treated quantum mechanically. These insights thus provide a foundation for the reliable investigation of spectroscopy and reactivity in aqueous environments.

Infrared and Raman Spectroscopy of Liquid Water through "First-Principles" Many-Body Molecular Dynamics.

Vibrational spectroscopy is a powerful technique to probe the structure and dynamics of water. However, deriving an unambiguous molecular-level interpretation of the experimental spectral features remains a challenge due to the complexity of the underlying hydrogen-bonding network. In this contribution, we present an integrated theoretical and computational framework (named many-body molecular dynamics or MB-MD) that, by systematically removing uncertainties associated with existing approaches, enables a rigorous modeling of vibrational spectra of water from quantum dynamical simulations. Specifically, we extend approaches used to model the many-body expansion of interaction energies to develop many-body representations of the dipole moment and polarizability of water. The combination of these "first-principles" representations with centroid molecular dynamics simulations enables the simulation of infrared and Raman spectra of liquid water under ambient conditions that, without relying on any ad hoc parameters, are in good agreement with the corresponding experimental results. Importantly, since the many-body energy, dipole, and polarizability surfaces employed in the simulations are derived independently from accurate fits to correlated electronic structure data, MB-MD allows for a systematic analysis of the calculated spectra in terms of both electronic and dynamical contributions. The present analysis suggests that, while MB-MD correctly reproduces both the shifts and the shapes of the main spectroscopic features, an improved description of quantum dynamical effects possibly combined with a dissociable water potential may be necessary for a quantitative representation of the OH stretch band.

Hydrogen Bonding and Related Properties in Liquid Water: A Car-Parrinello Molecular Dynamics Simulation Study.

The local hydrogen-bonding structure and dynamics of liquid water have been investigated using the Car-Parrinello molecular dynamics simulation technique. The radial distribution functions and coordination numbers around water molecules have been found to be strongly dependent on the number of hydrogen bonds formed by each molecule, revealing also the existence of local structural heterogeneities in the structure of the liquid. The results obtained have also revealed the strong effect of the local hydrogen-bonding network on the local tetrahedral structure and entropy. The investigation of the dynamics of the local hydrogen-bonding network in liquid water has shown that this network is very labile, and the hydrogen bonds break and reform very rapidly. Nevertheless, it has been found that the hydrogen-bonding states associated with the formation of four hydrogen bonds by a water molecule exhibit the largest survival probability and corresponding lifetime. The reorientational motions of water molecules have also been found to be strongly dependent on their initial hydrogen-bonding state. Finally, the dependence of the librational and vibrational modes of water molecules on the local hydrogen-bonding network has been carefully examined, revealing a significant effect upon the libration and bond-stretching peak frequencies. The calculated low frequency peaks come in agreement with previously reported interpretations of the experimental low-frequency Raman spectrum of liquid water.

Cloud and Aerosol Spectroscopy with Raman Lidar

Abstract A spectrometer for height-resolved measurements of the Raman backscatter-coefficient spectrum of water in its gaseous and condensed phases is presented. The spectrometer is fiber coupled to the far-range receiver of the Raman Lidar for Atmospheric Moisture Sensing (RAMSES) of the German Meteorological Service and consists of a Czerny–Turner spectrograph (500-mm focal length) and a 32-channel single-photon-counting detection system based on a multianode photomultiplier. During a typical measurement (transmitter wavelength of 355 nm), the spectrum between 385 and 410 nm is recorded with a spectral resolution of 0.79 nm; the vertical resolution is 15 m and the height range is 15 km. The techniques outlined are those that are applied to calibrate the spectrum measurement and to monitor fluorescence by atmospheric aerosols that have the potential to interfere with the water observation. For the first time, Raman spectra of liquid-water, mixed-phase, and cirrus clouds are reported, and their temperature dependence is investigated by means of band decomposition. The spectrum-integrated condensed-water Raman backscatter coefficient strongly depends on cloud particle volume, but it is not tightly correlated with the cloud optical properties (particle extinction and backscatter coefficient), which implies that retrieval of cloud water content from optical proxies is likely impossible. Aerosol measurements are also discussed. Depending on type, aerosols may show no backscattering in the spectrometer range at all, or a featureless spectrum that stems quite likely from fluorescence. Finally, the example of a cloud forming in an aerosol layer demonstrates that the new instrument not only opens up new perspectives in cloud research but also contributes to studies of cloud–aerosol interaction.

Raman Spectra of Liquid Water from Ab Initio Molecular Dynamics: Vibrational Signatures of Charge Fluctuations in the Hydrogen Bond Network.

We report the first ab initio simulations of the Raman spectra of liquid water, obtained by combining first principles molecular dynamics and density functional perturbation theory. Our computed spectra are in good agreement with experiments, especially in the low frequency region. We also describe a systematic strategy to analyze the Raman intensities, which is of general applicability to molecular solids and liquids, and it is based on maximally localized Wannier functions and effective molecular polarizabilities. Our analysis revealed the presence of intermolecular charge fluctuations accompanying the hydrogen bond (HB) stretching modes at 270 cm(-1), in spite of the absence of any Raman activity in the isotropic spectrum. We also found that charge fluctuations partly contribute to the 200 cm(-1) peak in the anisotropic spectrum, thus providing insight into the controversial origin of such peak. Our results highlighted the importance of taking into account electronic effects in interpreting the Raman spectra of liquid water and the key role of charge fluctuations within the HB network; they also pointed at the inaccuracies of models using constant molecular polarizabilities to describe the Raman response of liquid water.

Scanning cavity-enhanced droplet spectroscopy: Tuning of the excitation laser for obtaining a continuous Raman spectrum

Scanning cavity-enhanced droplet spectroscopy (S-CEDS) has been developed to obtain Raman spectra of liquid media with high sensitivity. Cavity-enhanced Raman spectra of a water droplet are measured by irradiation of a laser with nine different wavelengths. Each of the spectra consists of a series of intense and narrow peaks resulting from the cavity-enhancement conditions, and the envelope of their peak apexes displays a stimulated Raman spectrum of liquid water. A series of continuous Raman spectra of liquid water is obtained by scanning the laser wavelength, which dissolves the discreteness of each spectrum. S-CEDS provides a novel method to obtain stimulated Raman spectra of liquid media keeping their continuous spectral shapes.

Ab initio modeling of fused silica, crystal quartz, and water Raman spectra

Fused silica light-scattering properties are important for many industrial and laboratory applications. In this work, ab initio molecular dynamics employing periodic boundary conditions are used to simulate its structure and Raman scattering. Fused silica geometry is modeled as a frozen liquid, and the spectra are compared to those of crystal quartz. The method is tested also on Raman spectra of liquid water. Structural similarities between water and fused silica are discussed. The modeling agrees with experiment and suggests that most fused silica spectral features can be explained within the harmonic approximation. The combination of Raman spectroscopy with theoretical computations thus appears useful for structural studies of amorphous materials.

The Extreme Low‐Frequency Raman Spectrum of Liquid Water

Not merely sloshing around: The chemical and physical properties of water are very important. In contrast to its simplistic chemical formula, water possesses a complex dynamic behaviour (see picture). The bend, torsion, symmetric and asymmetric stretch modes of liquid water, close to the wavelength of the laser line have been predicted. Here, the low-frequency Raman spectrum of liquid water is presented confirming the presence of all four modes in the restricted translational region (35–300 cm−1).

Rattling the cage: Micro- to mesoscopic structure in liquids as simple as argon and as complicated as water

The water molecule has the convenient property that its molecular polarizability tensor is nearly isotropic while its dipole moment is large. As a result, the low-frequency anisotropic Raman spectrum of liquid water is mostly collision-induced and therefore reports primarily translational motions while the far-infrared (terahertz) and dielectric spectrum is dominated by rotational modes. Atomic and globular–molecular liquids have a zero dipole moment as well as an isotropic polarizability tensor. These spectrum-simplifying properties were exploited in a study of a number of liquids and solutions using ultrafast optical Kerr-effect (OKE) spectroscopy combined with dielectric relaxation spectroscopy (DRS), terahertz time-domain spectroscopy (THz-TDS), and terahertz field-induced second-harmonic generation (TFISH) spectroscopy. For room-temperature ionic liquids (RTILs), liquid water, aqueous salt solutions, noble gas liquids, and globular–molecular liquids it was found that, in each case, surprising structure and/or inhomogeneity is observed, ranging from mesoscopic clustering in RTILs to stretched-exponential dynamics in the noble gas liquids. For aqueous electrolyte solutions it is shown that the viscosity, normally described by the Jones–Dole expression, can be explained in terms of a jamming transition, a concept borrowed from soft condensed matter studies of glass transitions in colloidal suspensions.

Mechanism of the molecular mobility of water in the temperature range 298–359 K

The Raman spectra of liquid water in the region of O-H stretching vibrations were obtained in the temperature range 298–359 K. The Raman spectra were decomposed into the components using the XPSPEAK-4.1 program, and their temperature dependence was evaluated. The number of bifurcate hydrogen bonds and the percentage of rotational conformers containing bifurcate bonds were shown to increase with temperature. The defect mechanism of the molecular mobility of water on the hydrogen bond network in the temperature range 298–359 K was proposed.

IR and Raman spectra of liquid water: Theory and interpretation

IR and Raman (parallel- and perpendicular-polarized) spectra in the OH stretch region for liquid water were measured some years ago, but their interpretation is still controversial. In part, this is because theoretical calculation of such spectra for a neat liquid presents a formidable challenge due to the coupling between vibrational chromophores and the effects of motional narrowing. Recently we proposed an electronic structure/molecular dynamics method for calculating spectra of dilute HOD in liquid D(2)O, which relied on ab initio calculations on clusters to provide a map from nuclear coordinates of the molecules in the liquid to OH stretch frequencies, transition dipoles, and polarizabilities. Here we extend this approach to the calculation of couplings between chromophores. From the trajectories of the fluctuating local-mode frequencies, transition moments, and couplings, we use our recently developed time-averaging approximation to calculate the line shapes. Our results are in good agreement with experiment for the IR and Raman line shapes, and capture the significant differences among them. Our analysis shows that while the coupling between chromophores is relatively modest, it nevertheless produces delocalization of the vibrational eigenstates over up to 12 chromophores, which has a profound effect on the spectroscopy. In particular, our results demonstrate that the peak in the parallel-polarized Raman spectrum at about 3250 wavenumbers is collective in nature.

On the calculations of the vibrational Raman spectra of small water clusters

The static and frequency-dependent Raman intensities for small water clusters ((H 2O) n , n = 2–8) were calculated by means of the DFT(B3LYP)/d-aug-cc-pVDZ method. The obtained intensities were analyzed in order to find a correlation between the topology of a cluster and its calculated Raman spectrum. A change from 170 for water dimer to 677 Å 4 u −1 for hexamer-ring cluster reveals a tendency of a practically linear increase in the Raman intensity of the lowest-frequency symmetric OH stretching mode with a number of water molecules in cyclic clusters (including dimer). The stretching vibrations of the three-coordinated single donor–double acceptor (DAA) molecules are marked by higher Raman intensities than the double donor–single acceptor (DDA) molecules. The simple average Raman intensity over the stretching modes of the DAA molecules yields 110 Å 4 u −1 while the corresponding value for the DDA molecules is 70 Å 4 u −1. The Raman intensities calculated at 351.1 nm are larger by 10–40% than the static ones. Previously suggested in literature origin of the shoulder on the blue side of the Raman spectrum of liquid water as arising from the water molecules whose protons are not engaged to hydrogen bond can be confirmed by our calculations.