Development of single-channel heterodyne-detected sum frequency generation spectroscopy and its application to the water/vapor interface

Water orientation on platinum surfaces controlled by step sites

In this work, the adsorption structure of deuterated water on the stepped platinum surface is studied under an ultra-high vacuum by using heterodyne-detected sum-frequency generation spectroscopy. On a pristine Pt(553), D2O molecules adsorbed at the step sites act as hydrogen bond (H-bond) donors to the adjacent terrace sites. This ensures the net D-down orientation at the terrace sites away from the steps. In particular, the pre-adsorption of oxygen atoms at the step sites significantly alters the D-down configuration. The oxygen pre-adsorption leads to a spontaneous dissociation of the post-adsorbed water molecules at the step to form hydroxyl (OD) species. Since the hydroxyl at the step acts as a strong H-bond acceptor, D2O at the terrace no longer maintains the D-down configuration and adopts flat-lying configurations, significantly reducing the number of D-down molecules at the terrace. Density-functional theoretical calculations support these pictures. This work demonstrates the critical role of steps in controlling the net orientation of the interfacial water and provides an important reference for future considerations of the reactions at electrochemical interfaces.

Phase of the second-order susceptibility in vibrational sum frequency generation spectroscopy: Origins, utility, and measurement techniques

Vibrational sum frequency generation can provide valuable structural information at surfaces and buried interfaces. Relating the measured spectra to the complex-valued second-order susceptibility χ(2) is at the heart of the technique and a requisite step in nearly all subsequent analyses. The magnitude and phase of χ(2) as a function of frequency reveal important information about molecules and materials in regions where centrosymmetry is broken. In this tutorial-style perspective, the origins of the χ(2) phase are first described, followed by the utility of phase determination. Finally, some practical methods of phase extraction are discussed.

Sum rule comparison of narrowband and broadband sum frequency generation spectra and comparison with theory

Earlier sum frequency generation (SFG) experiments involve one infrared and one visible laser, and a measurement of the intensity of the response, yielding data on the surface sensitive properties of the sample. Recently, both the real and imaginary components of the susceptibility were measured in two different sets of experiments. In one set, a broadband infrared laser was used, permitting observations at very short times, while in another set the infrared laser was narrowband, permitting higher spectral resolution. The differences in the spectrum obtained by the two will be most evident in studying narrow absorption bands, e.g., the band due to dangling OH bonds at a water interface. The direct comparisons in the integrated amplitude (sum rule) of the imaginary part of the dangling OH bond region differ by a factor of 3. Due to variations in experimental setup and data processing, corrections were made for the quartz reference, Fresnel factors, and the incident visible laser wavelength. After the corrections, the agreement differs now by the factors of 1.1 within broadband and narrowband groups and the two groups now differ by a factor of 1.5. The 1.5 factor may arise from the extra heating of the more powerful broadband laser system on the water surface. The convolution from the narrowband SFG spectrum to the broadband SFG spectrum is also investigated and it does not affect the sum rule. Theory and narrowband experiments are compared using the sum rule and agree to a factor of 1.3 with no adjustable parameters.

Theoretical Study of the Two-Dimensional Vibrational Sum Frequency Generation Spectroscopy of the Air-Water Interface at Varying Temperature and Its Connections to the Interfacial Structure and Dynamics

We performed a theoretical study of the temperature variation of two-dimensional vibrational sum frequency generation (2D-VSFG) spectra of the OH stretch modes at air-water interfaces in the mid-IR region. The calculations are performed at four different temperatures from 250 to 325 K by using a combination of techniques involving response function formalism of nonlinear spectroscopy, electronic structure calculations, and molecular dynamics simulations. Also, the calculations are performed for isotopically dilute solutions so that the intra- and intermolecular coupling between the vibrational modes of interest can be ignored. We have established the connections of temperature variation of various frequency- and time-dependent features of the calculated spectra to the changes in the underlying structure and dynamics of the interfaces. The results reveal that interfacial water is dynamically more heterogeneous than bulk water, with three dominant dynamical processes exhibiting their corresponding time-dependent features in the 2D-VSFG spectrum. These are the spectral diffusion of hydrogen-bonded OH groups at the interface, conversion of an initially hydrogen-bonded OH group to a dangling OH which is a stable state for surface water, unlike the bulk water, and the third one, which involves the conversion of an initially free or dangling OH group to its hydrogen-bonded state at the interface. The temporal appearance of the cross peaks corresponding to interconversion of the hydrogen-bonded state to the dangling state or vice versa of an interfacial OH group is found to take place at a slower rate than the dynamics of spectral diffusion of hydrogen-bonded molecules at the interface, which, in turn, is slower than the corresponding spectral diffusion of bulk water molecules. The temperature variation of these dynamic processes can be linked to the decay of appropriate hydrogen-bond and non-hydrogen-bond time correlation functions of interfacial water molecules for the different air-water systems studied in this work.

Appraisal of TIP4P-type models at water surface

In view of the current situation in which non-polarizable rigid water models have been scarcely examined against surface-specific properties, we appraise TIP4P-type models at the liquid water surface on the basis of heterodyne-detected sum frequency generation (HD-SFG) spectroscopy. We find in the HD-SFG spectrum of the water surface that the peak frequency of the hydrogen-bonded OH band, the half width at half maximum of the hydrogen-bonded OH band, and the full width at half maximum of the free OH band are best reproduced by TIP4P, TIP4P/Ew, and TIP4P/Ice, respectively, whereas it is already well known that TIP4P/2005 best reproduces the surface tension. These TIP4P-type models perform better at the water surface in terms of the present appraisal items than some polarizable models in the literature.

Transferability of vibrational spectroscopic map from TIP4P to TIP4P-like water models

We computed the IR, Raman, and sum frequency generation spectra of water in the OH-stretch region by employing the quantum/classical mixed approach that consists of a vibrational spectroscopic map and molecular dynamics (MD) simulation. We carried out the MD simulation with the TIP4P, TIP4P/2005, and TIP4P/Ice models and applied the map designed for TIP4P by Skinner et al. to each MD trajectory. Although the map is not tuned for TIP4P-like models, TIP4P/2005 and TIP4P/Ice provide the best reproduction of the experimental vibrational spectra of liquid water and crystalline ice, respectively. This result demonstrates the transferability of the map from TIP4P to TIP4P/2005 and TIP4P/Ice, meaning that one can choose an appropriate TIP4P-like model to calculate the vibrational spectra of an aqueous system without rebuilding the map.

Experimental and Theoretical Heterodyne-Detected Sum Frequency Generation Spectroscopy of Isotopically Pure and Diluted Water Surfaces

The χ⁽²⁾ (second-order nonlinear optical susceptibility) spectrum of the water surface has been a matter of debate for a few decades. Here, we report that we experimentally measured the isotopic dilution dependence of the χ⁽²⁾ spectrum and theoretically reproduced it by employing the quantum/classical mixed approach with a new idea to subtract an artifact. The present theoretical framework allows for clarifying the effects of the intramolecular, intermolecular, and Fermi resonance couplings on the OH-stretch vibrational spectra of water at the surface as well as in the bulk.

Proton Configuration in Water Chain on Pt(533)

In this paper, a wetting behavior of Pt(533) is studied by using heterodyne-detected vibrational sum-frequency generation spectroscopy under an ultrahigh-vacuum condition at 145 K. The imaginary parts of the surface nonlinear susceptibility (Imχ⁽²⁾) of the H-bonded OH stretching region are successfully obtained for submonolayer water coverage that show negative bands indicating H-down (proton pointing to the substrate) configurations both for the water at the step and at the terrace. The growth manner of the Imχ⁽²⁾ signal with coverage and the results of an isotopic dilution are consistent with a model in which a one-dimensional (1D) chain at the step forms a "zigzag" structure that contains H-down orientations. This finding resolves the previous controversy in the literature concerning the proton configuration in the 1D water chain at the step.

Polarization-Dependent Heterodyne-Detected Sum-Frequency Generation Spectroscopy as a Tool to Explore Surface Molecular Orientation and Ångström-Scale Depth Profiling

Sum-frequency generation (SFG) spectroscopy provides a unique optical probe for interfacial molecules with interface-specificity and molecular specificity. SFG measurements can be further carried out at different polarization combinations, but the target of the polarization-dependent SFG is conventionally limited to investigating the molecular orientation. Here, we explore the possibility of polarization-dependent SFG (PD-SFG) measurements with heterodyne detection (HD-PD-SFG). We stress that HD-PD-SFG enables accurate determination of the peak amplitude, a key factor of the PD-SFG data. Subsequently, we outline that HD-PD-SFG can be used not only for estimating the molecular orientation but also for investigating the interfacial dielectric profile and studying the depth profile of molecules. We further illustrate the variety of combined simulation and PD-SFG studies.

Ultrafast vibrational dynamics of the free OD at the air/water interface: Negligible isotopic dilution effect but large isotope substitution effect

Vibrational relaxation dynamics of the OH stretch of water at the air/water interface has been a subject of intensive research, facilitated by recent developments in ultrafast interface-selective nonlinear spectroscopy. However, a reliable determination of the vibrational relaxation dynamics in the OD stretch region at the air/D₂O interface has not been yet achieved. Here, we report a study of the vibrational relaxation of the free OD carried out by time-resolved heterodyne-detected vibrational sum frequency generation spectroscopy. The results obtained with the aid of singular value decomposition analysis indicate that the vibrational relaxation (T₁) time of the free OD at the air/D₂O interface and air/isotopically diluted water (HOD-H₂O) interfaces show no detectable isotopic dilution effect within the experimental error, as in the case of the free OH in the OH stretch region. Thus, it is concluded that the relaxation of the excited free OH/OD predominantly proceeds with their reorientation, negating a major contribution of the intramolecular energy transfer. It is also shown that the T₁ time of the free OD is substantially longer than that of the free OH, further supporting the reorientation relaxation mechanism. The large difference in the T₁ time between the free OD and the free OH (factor of ∼2) may indicate the nuclear quantum effect on the diffusive reorientation of the free OD/OH because this difference is significantly larger than the value expected for a classical rotational motion.

Choose your own adventure: Picosecond or broadband vibrational sum-frequency generation spectroscopy

Vibrational sum-frequency generation (VSFG) spectroscopy is a method capable of measuring chemical structure and dynamics within the interfacial region between two bulk phases. At the core of every experimental system is a laser source that influences the experimental capabilities of the VSFG spectrometer. In this article, we discuss the differences between VSFG spectrometers built with picosecond and broadband laser sources as it will impact everything from material costs, experimental build time, experimental capabilities, and more. A focus is placed on the accessibility of the two different SFG systems to newcomers in the SFG field and provides a resource for laboratories considering incorporating VSFG spectroscopy into their research programs. This Tutorial provides a model decision tree to aid newcomers when determining whether the picosecond or femtosecond laser system is sufficient for their research program and navigates through it for a few specific scenarios.

Passively Stabilized Phase-Resolved Collinear SFG Spectroscopy Using a Displaced Sagnac Interferometer

Sum-frequency generation (SFG) vibrational spectroscopy is a powerful technique to study interfaces at the molecular level. Phase-resolved SFG (PR-SFG) spectroscopy provides direct information on interfacial molecules' orientation. However, its implementation is technologically demanding: it requires the generation of a local oscillator wave and control of its time delay with sub-fs accuracy. Commonly used noncollinear PR-SFG provides this control naturally but requires very accurate sample height control. Collinear PR-SFG spectroscopy is less demanding regarding sample positioning, but tuning the local oscillator time delay with this beam geometry is challenging. Here, we develop a collinear PR-SFG setup using a displaced Sagnac interferometer. This scheme allows full, independent control of the time delay and intensity of the local oscillator and provides long-time phase stabilization (better than 5° over 12 h) for the measured signal. This approach substantially reduces the complexity of an experimental setup and combines the advantages of collinear and noncollinear PR-SFG techniques.

Energy relaxation dynamics of hydrogen-bonded OH vibration conjugated with free OH bond at an air/water interface

Vibrational energy relaxation dynamics of the excited hydrogen-bonded (H-bonded) OH conjugated with free OH (OD) at an air/water (for both pure water and isotopically diluted water) interface are elucidated via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulations. The calculated results are compared with those of the excited H-bonded OH in bulk liquid water reported previously. In the case of pure water, the relaxation timescale (vibrational lifetime) of the excited H-bonded OH at the interface is T₁ = 0.13 ps, which is slightly larger than that in the bulk (T₁ = 0.11 ps). Conversely, in the case of isotopically diluted water, the relaxation timescale of T₁ = 0.74 ps in the bulk decreases to T₁ = 0.26 ps at the interface, suggesting that the relaxation dynamics of the H-bonded OH are strongly dependent on the surrounding H-bond environments particularly for the isotopically diluted conditions. The relaxation paths and their rates are estimated by introducing certain constraints on the vibrational modes except for the target path in the NE-AIMD simulation to decompose the total energy relaxation rate into contributions to possible relaxation pathways. It is found that the main relaxation pathway in the case of pure water is due to intermolecular OH⋯OH vibrational coupling, which is similar to the relaxation in the bulk. In the case of isotopically diluted water, the main pathway is due to intramolecular stretch and bend couplings, which show more efficient relaxation than in the bulk because of strong H-bonding interactions specific to the air/water interface.

Progress in phase-sensitive sum frequency generation spectroscopy

Sum frequency generation (SFG) spectroscopy is a unique and powerful tool for investigating surfaces and interfaces at the molecular level. Phase-sensitive SFG (PS-SFG) is an upgraded technique that can overcome the inherent drawbacks of conventional SFG. Here we review several methods of PS-SFG developed and reported in 1990-2020. We introduce how and by which group each PS-SFG method was designed and built in terms of interferometer implementation for optical heterodyne detection, with one exception of a recent numerical method that does not rely on interferometry. We also discuss how PS-SFG solved some typical problems for aqueous interfaces that were once left open by conventional SFG. These problems and their solutions are good examples to demonstrate why PS-SFG is essential. In addition, we briefly note a few terminology issues related with PS-SFG to avoid confusion.

Kosmotropic Electrolyte (Na2CO3, NaF) Perturbs the Air/Water Interface through Anion Hydration Shell without Forming a Well-Defined Electric Double Layer

The ion-driven electric double layer (EDL) and the structural transformation of interfacial water are implicated in unusual reaction kinetics at the air/water interface. By combining heterodyne-detected vibrational sum frequency generation (HD-VSFG) with differential spectroscopy involving simultaneous curve fitting (DS-SCF) analysis, we retrieve electrolyte (Na₂CO₃ and NaF)-correlated OH-stretch spectra of water at the air/water interface. Vibrational mapping of the perturbed interfacial water with the hydration shell spectra (obtained by DS-SCF analysis of Raman spectra) of the corresponding anion discloses that the kosmotropic electrolytes do not form well-defined EDL at the air/water interface. Instead, the interfacial water forms a stronger hydrogen-bond with the surface-expelled anions (CO₃^2- and F^-) and becomes more inhomogeneous than the pristine air/water interface. Together, the results reveal that the perturbation of interfacial water by the kosmotropic electrolyte is a "local phenomenon" confined within the hydration shell of the surface-expelled anion.

"Breaking" and "Making" of Water Structure at the Air/Water-Electrolyte (NaXO3; X = Cl, Br, I) Interface

The prevalence of ions at the aqueous interface has been widely recognized, but their effect on the structure of interfacial water (e.g., hydrogen (H)-bonding) remains enigmatic. Using heterodyne-detected vibrational sum frequency generation (HD-VSFG) and Raman difference spectroscopy with simultaneous curve fitting (DS-SCF) analysis, we show that the ion-induced perturbations of H-bonding at the air/water interface and in the bulk water are strongly correlated. Specifically, the structure-breaking anions such as ClO₃^- decrease the average H-bonding of water at the air/water interface, as it does to the water in its hydration shell in the bulk. The structure-making anion of the same series (IO₃^-) does exactly the reverse. None of the electrolytes (NaXO₃; X = Cl, Br, I) form well-defined electric double layers that significantly increase or reverse the hydrogen-down (H-down) orientation of water at the air/water interface. These results provide a unified picture of specific anion effect at the air/water interface and in the bulk water.

Interaction of α-Synuclein with Phospholipids and the Associated Restructuring of Interfacial Lipid Water: An Interface-Selective Vibrational Spectroscopic Study

Interaction of α-Synuclein (αS) with biological lipids is crucial for the onset of its fibrillation at the cell membrane/water interface. Probed herein is the interaction of αS with membrane-mimicking lipid monolayer/water interfaces. The results depict that αS interacts negligibly with zwitterionic lipids, but strongly affects the pristine air/water and charged lipid/water interfaces by perturbing the structure and orientation of the interfacial water. The net negative αS (-9 in bulk water; pH 7.4) reorients the water as hydrogen-up (H-up) at the air/water interface, and electrostatically interacts with positively charged lipids, making the interface nearly net neutral. αS also interacts with negatively charged lipids: the net H-up orientation of the interfacial water decreases at the anionic lipid/water interface, revealing a domain-specific interaction of net negative αS with the negatively charged lipids at the membrane surface.

Resolving the Controversy over Dipole versus Quadrupole Mechanism of Bend Vibration of Water in Vibrational Sum Frequency Generation Spectra

Recently, there has been controversy over whether the HOH bend signal of water in the vibrational sum frequency generation (VSFG) spectrum arises from the conventional dipole mechanism or the quadrupole mechanism. Here, we show that the Im χ⁽²⁾ (the imaginary part of the second-order nonlinear susceptibility) spectra of the HOH bend mode of water at oppositely charged monolayer/water interfaces all exhibit positive bands, irrespective of the difference in the sign of the charge at the interface. Furthermore, it is found that the peak frequency of the HOH bend band substantially changes depending on the chemical structure of the charged headgroup located at the interface. These results demonstrate that the VSFG signal of the HOH bend vibration is generated from interfacial water with the interfacial quadrupole mechanism that is associated with the large field gradient of incident lights localized in a very thin region at the interface.

Adsorption of Iodine Species (I3-, I-, and IO3-) at the Nuclear Paint Monolayer-Water Interface and Its Relevance to a Nuclear Accident Scenario

Following a nuclear accident, radioactive iodine causes great concern to public health and safety. Organic iodide, because of its ability to escape reactor containment building and high environmental mobility, constitutes a predominant fraction of airborne radioiodine at places far away from the accident site. As the iodine released from a reactor core is inorganic iodine, it is vital to understand the mechanism of organic iodide formation inside reactor containment. In this context, we investigated the surface prevalence and adsorption of various inorganic iodines, I^-, I₃^-, and IO₃^-, at a nuclear paint (used in nuclear installations) monolayer-water interface, mimicking the painted inner walls of an accident-affected containment building that are exposed to the iodine-containing condensed water layer. Vibrational sum frequency generation (VSFG) measurements in the OH and CH stretch regions reveal that the paint-water interface changes its charge characteristics with the pH of the water that affects the degree of interaction with the iodine species. At the acidic condition (bulk pH < 7), the paint becomes positively charged and strongly adsorbs the negatively charged iodine species dissolved in the aqueous phase, whereas at the alkaline condition (bulk pH > 9.5), the paint becomes net neutral and weakly interacts with the iodine species. These interactions change the conformation of the paint such that its hydrophobic alkyl groups orient increasingly away from the aqueous phase. The order of adsorption increases as IO₃^- < I^- < I₃^- for the different iodine species studied.

Molecular Structure and Modeling of Water-Air and Ice-Air Interfaces Monitored by Sum-Frequency Generation

From a glass of water to glaciers in Antarctica, water-air and ice-air interfaces are abundant on Earth. Molecular-level structure and dynamics at these interfaces are key for understanding many chemical/physical/atmospheric processes including the slipperiness of ice surfaces, the surface tension of water, and evaporation/sublimation of water. Sum-frequency generation (SFG) spectroscopy is a powerful tool to probe the molecular-level structure of these interfaces because SFG can specifically probe the topmost interfacial water molecules separately from the bulk and is sensitive to molecular conformation. Nevertheless, experimental SFG has several limitations. For example, SFG cannot provide information on the depth of the interface and how the orientation of the molecules varies with distance from the surface. By combining the SFG spectroscopy with simulation techniques, one can directly compare the experimental data with the simulated SFG spectra, allowing us to unveil the molecular-level structure of water-air and ice-air interfaces. Here, we present an overview of the different simulation protocols available for SFG spectra calculations. We systematically compare the SFG spectra computed with different approaches, revealing the advantages and disadvantages of the different methods. Furthermore, we account for the findings through combined SFG experiments and simulations and provide future challenges for SFG experiments and simulations at different aqueous interfaces.

Hydrogen Bonds and Molecular Orientations of Supramolecular Structure between Barbituric Acid and Melamine Derivative at the Air/Water Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

We studied the supramolecular structure between barbituric acid (pyrimidine-2,4,6(1H,3H,5H)-trione, BA) and an amphiphilic melamine derivative at the air/water interface by heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. HD-VSFG measurements in situ showed a positive broad band from 2300 to 2950 cm^-1. By comparing the experimental results with ab initio molecular dynamics (AIMD) simulations, we assigned the broad band to the NH stretching modes of BA strongly hydrogen-bonded to the melamine derivative. In addition, we report in situ HD-VSFG spectra of the interfacial supramolecular structure in the CO stretching region. Two CO stretching bands were identified. On the basis of the signs of the C=O bands, we uniquely determined the orientation of BA. The strong hydrogen bonds and the molecular orientations are direct evidence for the supramolecular structure based on complementary hydrogen bonds at the air/water interface.

Sum frequency generation, calculation of absolute intensities, comparison with experiments, and two-field relaxation-based derivation

The experimental sum frequency generation (SFG) spectrum is the response to an infrared pulse and a visible pulse and is a highly surface-sensitive technique. We treat the surface dangling OH bonds at the air/water interface and focus on the absolute SFG intensities for the resonant terms, a focus that permits insight into the consequences of some approximations. For the polarization combinations, the calculated linewidths for the water interface dangling OH SFG band at 3,700 [Formula: see text] are, as usual, too large, because of the customary neglect of motional narrowing. The integrated spectrum is used to circumvent this problem and justified here using a Kubo-like formalism and theoretical integrated band intensities rather than peak intensities. Only relative SFG intensities are usually reported. The absolute integrated SFG intensities for three polarization combinations for sum frequency, visible, and infrared beams are computed. We use molecular dynamics and the dipole and the polarizability matrix elements obtained from infrared and Raman studies of [Formula: see text]O vapor. The theoretical expressions for two of the absolute susceptibilities contain only a single term and agree with experiment to about a factor of 1.3, with no adjustable parameters. The Fresnel factors are included in that comparison. One of the susceptibilities contains instead four positive and negative terms and agrees less well. The expression for the SFG correlation function is normally derived from a statistical mechanical formulation using a time-evolving density matrix. We show how a derivation based on a two-field relaxation leads to the same final result.

A hydrogen-bonding structure in self-formed nanodroplets of water adsorbed on amorphous silica revealed via surface-selective vibrational spectroscopy

Water adsorption onto a material surface is known to change macroscopic surface properties such as wettability and friction coefficient.

Unveiling Heterogeneity of Interfacial Water through the Water Bending Mode

The water bending mode provides a powerful probe of the microscopic structure of bulk aqueous systems because its frequency and spectral line shape are responsive to the intermolecular interactions. Furthermore, interpreting the bending mode response is straightforward, as the intramolecular vibrational coupling is absent. Nevertheless, bending mode has not been used for probing the interfacial water structure, as it has been yet argued that the signal is dominated by bulk effects. Here, through the sum-frequency generation measurement of the water bending mode at the water/air and water/charged lipid interfaces, we demonstrate that the bending mode signal is dominated not by the bulk but by the interface. Subsequently, we disentangle the hydrogen-bonding of water at the water/air interface using the bending mode frequency distribution and find distinct interfacial hydrogen-bonded structures, which can be directly related to the interfacial organization of water. The bending mode thus provides an excellent probe of aqueous interfacial structure.

Nuclear Quantum Effect on the χ(2) Band Shape of Vibrational Sum Frequency Generation Spectra of Normal and Deuterated Water Surfaces

In the vibrational sum frequency generation (VSFG) spectrum of the air/water interface, there is an open question whether the imaginary spectrum of nonlinear susceptibility Im[χ⁽²⁾] has a positive band in the low-frequency tail of the OH stretching band. We previously elucidated by molecular dynamics (MD) analysis of the VSFG spectrum that the positive band could arise from particularly strong hydrogen-bonded water pairs at the water surface. This mechanism should be emphasized in OD stretching in comparison to OH, because OD forms stronger hydrogen bonds. Therefore, we calculated the Im[χ⁽²⁾] spectra of normal and deuterated water surfaces by MD simulation including the nuclear quantum effect and demonstrated that the low-frequency positive feature could arise in the tail of the OD stretching band. This positive feature is sensitive to the oxygen-oxygen distance of hydrogen-bonding pairs at the surface and hence the temperature and disappears with increasing temperature.

Accessing the Accuracy of Density Functional Theory through Structure and Dynamics of the Water-Air Interface

Density functional theory-based molecular dynamics simulations are increasingly being used for simulating aqueous interfaces. Nonetheless, the choice of the appropriate density functional, critically affecting the outcome of the simulation, has remained arbitrary. Here, we assess the performance of various exchange-correlation (XC) functionals, based on the metrics relevant to sum-frequency generation spectroscopy. The structure and dynamics of water at the water-air interface are governed by heterogeneous intermolecular interactions, thereby providing a critical benchmark for XC functionals. We find that the XC functionals constrained by exact functional conditions (revPBE and revPBE0) with the dispersion correction show excellent performance. The poor performance of the empirically optimized density functional (M06-L) indicates the importance of satisfying the exact functional condition. Understanding the performance of different XC functionals can aid in resolving the controversial interpretation of the interfacial water structure and direct the design of novel, improved XC functionals better suited to describing the heterogeneous interactions in condensed phases.

Polyatomic Iodine Species at the Air-Water Interface and Its Relevance to Atmospheric Iodine Chemistry: An HD-VSFG and Raman-MCR Study

Iodine plays a key role in tropospheric ozone destruction, atmospheric new particle formation, as well as growth. Air-water interface happens to be an important reaction site pertaining to such phenomena. However, except iodide (I^-), the behavior of other iodine species, for example, triiodide (I₃^-) and iodate (IO₃^-, the most abundant iodine species in seawater) at the aqueous interface and their effect on the interfacial water are largely unknown. Using interface-specific vibrational spectroscopy (heterodyne-detected vibrational sum frequency generation), we recorded the imaginary-χ⁽²⁾ spectra (Imχ⁽²⁾; χ⁽²⁾ is the second-order electric susceptibility in OH stretch region) of the air-water interface in the presence of IO₃^-, I₃^-, and I^- (≤0.3 M) in the aqueous subphase. The Imχ⁽²⁾ spectra reveal that the chaotropic I₃^- is the most surface-active anion among the iodine species studied and decreases the vibrational coupling and hydrogen-bonding of interfacial water. Interestingly, the IO₃^-, even being a kosmotrope, is quite prevalent in the interfacial region and preferentially orients the interfacial water as "H-down" (i.e., water dipole moment is pointed toward the bulk water). Mapping of the OH stretch response of ion-affected water at interface (i.e., ΔImχ⁽²⁾ = Imχ⁽²⁾_{air-water-iodine salt} - Imχ⁽²⁾_air-water) with that in the hydration shell of the respective ion (hydration shell water response is obtained by Raman multivariate curve resolution spectroscopy) reveals a correlative link between the ion's influence on the interfacial water and their hydration shell structure. The distinct water structure of stronger as well as weaker H-bonding in the hydration shell of the polyatomic IO₃^- anion promotes the anion to stay at the interfacial region. Thus, the surface prevalence of the iodine species and their effect on the interfacial water are perceived to be crucial for the transfer of iodine from seawater to the atmosphere across the marine boundary layer and the chemistry of iodine at aqueous aerosol surface.

Phase-Sensitive Sum-Frequency Generation Measurements Using a Femtosecond Nonlinear Interferometer

Phase-sensitive sum-frequency spectroscopy is a unique tool to interrogate the vibrational structure of interfaces. A precise understanding of the interfacial structure often relies on accurately determining the phase of χ⁽²⁾, which has recently been demonstrated using a nonlinear interferometer in conjunction with a frequency-scanning picosecond laser system. Here, we implement nonlinear interferometry using a femtosecond laser system for broadband sum-frequency generation. The phase of the vibrational response from a self-assembled monolayer of octadecanethiol on gold is determined using the nonlinear femtosecond interferometer. The results are compared to those obtained using the more traditional heterodyne-detected phase measurements. Both methods give a similar phase spectrum and phase uncertainty. We also discuss the origin of the phase uncertainties and provide guidelines for further improvement.

Ab initio Modeling of the Vibrational Sum-Frequency Generation Spectrum of Interfacial Water

Understanding the structural and dynamical features of interfacial water is of greatest interest in physics, chemistry, biology, and materials science. Vibrational sum-frequency generation (SFG) spectroscopy, which is sensitive to the molecular orientation and dynamics on the surfaces or at the interfaces, allows one to study a wide variety of interfacial systems. The structural and dynamical features of interfacial water at the air/water interface have been extensively investigated by SFG spectroscopy. However, the interpretations of the spectroscopic features have been under intense debate. Here, we report a simulated SFG spectrum of the air/water interface based on ab initio molecular dynamics simulations, which covers the OH stretching, bending, and libration modes of interfacial water. Quantitative agreement between our present simulations and the most recent experimental studies ensures that ab initio simulations predict unbiased structural features and electrical properties of interfacial systems. By utilizing the kinetic energy spectral density (KESD) analysis to decompose the simulated spectra, the spectroscopic features can then be assigned to specific hydrogen-bonding configurations of interfacial water molecules.

Propofol adsorption at the air/water interface: a combined vibrational sum frequency spectroscopy, nuclear magnetic resonance and neutron reflectometry study

Propofol is an amphiphilic small molecule that strongly influences the function of cell membranes, yet data regarding interfacial properties of propofol remain scarce. Here we consider propofol adsorption at the air/water interface as elucidated by means of vibrational sum frequency spectroscopy (VSFS), neutron reflectometry (NR), and surface tensiometry. VSFS data show that propofol adsorbed at the air/water interface interacts with water strongly in terms of hydrogen bonding and weakly in the proximity of the hydrocarbon parts of the molecule. In the concentration range studied there is almost no change in the orientation adopted at the interface. Data from NR show that propofol forms a dense monolayer with a thickness of 8.4 Å and a limiting area per molecule of 40 Å2, close to the value extracted from surface tensiometry. The possibility that islands or multilayers of propofol form at the air/water interface is therefore excluded as long as the solubility limit is not exceeded. Additionally, measurements of the 1H NMR chemical shifts demonstrate that propofol does not form dimers or multimers in bulk water up to the solubility limit.

Orientational Distribution of Free O-H Groups of Interfacial Water is Exponential

The orientational distribution of free O-H (O-D) groups at the H_{2}O- (D_{2}O-)air interface is investigated using combined molecular dynamics (MD) simulations and sum-frequency generation (SFG) experiments. The average angle of the free O-H groups, relative to the surface normal, is found to be ∼63°, substantially larger than previous estimates of 30°-40°. This discrepancy can be traced to erroneously assumed Gaussian or stepwise orientational distributions of free O-H groups. Instead, the MD simulation and SFG measurement reveal a broad and exponentially decaying orientational distribution. The broad orientational distribution indicates the presence of the free O-H group pointing down to the bulk. We ascribe the origin of such free O-H groups to the presence of capillary waves on the water surface.

Spectroscopic BIL-SFG Invariance Hides the Chaotropic Effect of Protons at the Air-Water Interface

The knowledge of the water structure at the interface with the air in acidic pH conditions is of utmost importance for chemistry in the atmosphere. We shed light on the acidic air-water (AW) interfacial structure by DFT-MD simulations of the interface containing one hydronium ion coupled with theoretical SFG (Sum Frequency Generation) spectroscopy. The interpretation of SFG spectra at charged interfaces requires a deconvolution of the signal into BIL (Binding Interfacial Layer) and DL (Diffuse Layer) SFG contributions, which is achieved here, and hence reveals that even though H 3 O + has a chaotropic effect on the BIL water structure (by weakening the 2D-HBond-Network observed at the neat air-water interface) it has no direct probing in SFG spectroscopy. The changes observed experimentally in the SFG of the acidic AW interface from the SFG at the neat AW are shown here to be solely due to the DL-SFG contribution to the spectroscopy. Such BIL-SFG and DL-SFG deconvolution rationalizes the experimental SFG data in the literature, while the hydronium chaotropic effect on the water 2D-HBond-Network in the BIL can be put in perspective of the decrease in surface tension at acidic AW interfaces.

Molecular dynamics study of structure and vibrational spectra at zwitterionoic lipid/aqueous KCl, NaCl, and CaCl2 solution interfaces

Molecular dynamics (MD) simulations of KCl, NaCl, and CaCl₂ solution/dipalmytoylphosphatidylcholine lipid interfaces were performed to analyze heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectra in relation to the interfacial water structure. The present MD simulation well reproduces the experimental spectra and elucidates a specific cation effect on the interfacial structure. The K⁺, Na⁺, and Ca²⁺ cation species penetrate in the lipid layer more than the anions in this order, due to the electrostatic interaction with negative polar groups of lipid, and the electric double layer between the cations and anions cancels the intrinsic orientation of water at the water/lipid interface. These mechanisms explain the HD-VSFG spectrum of the water/lipid interface and its spectral perturbation by adding the ions. The lipid monolayer reverses the order of surface preference of the cations at the solution/lipid interface from that at the solution/air interface.

Structure and dynamics of water at water-graphene and water-hexagonal boron-nitride sheet interfaces revealed by ab initio sum-frequency generation spectroscopy

We simulate sum-frequency generation (SFG) spectra of isotopically diluted water at the water-graphene and water-hexagonal boron-nitride (hBN) sheet interfaces, using ab initio molecular dynamics simulations. A sharp 'dangling' O-D peak around ∼2640 cm-1 appearing in both simulated SFG spectra evidences that both graphene and hBN are hydrophobic. The dangling O-D peak is 10 cm-1 red-shifted at the water-hBN interface relative to the peak at the water-graphene interface. This frequency difference gives a stronger O-DN intermolecular interaction between water and hBN than the O-DC interaction between water and graphene. Accordingly, the anisotropy decay of such a dangling O-D group slows down near hBN compared with near graphene, illustrating that the dynamics of the dangling O-D group are also affected by the stronger O-DN interaction than the O-DC interaction. We discuss molecular-level insights into the structure and dynamics of interfacial water in the context of the friction of hBN and graphene.

Second-Order Vibrational Lineshapes from the Air/Water Interface

We explore by means of modeling how absorptive-dispersive mixing between the second- and third-order terms modifies the imaginary χ_total⁽²⁾ responses from air/water interfaces under conditions of varying charge densities and ionic strength. To do so, we use published Im(χ⁽²⁾) and χ⁽³⁾ spectra of the neat air/water interface that were obtained either from computations or experiments. We find that the χ_total⁽²⁾ spectral lineshapes corresponding to experimentally measured spectra contain significant contributions from both interfacial χ⁽²⁾ and bulk χ⁽³⁾ terms at interfacial charge densities equivalent to less than 0.005% of a monolayer of water molecules, especially in the 3100 to 3300 cm^-1 frequency region. Additionally, the role of short-range static dipole potentials is examined under conditions mimicking brine. Our results indicate that surface potentials, if indeed present at the air/water interface, manifest themselves spectroscopically in the tightly bonded H-bond network observable in the 3200 cm^-1 frequency range.

Water at surfaces with tunable surface chemistries

Aqueous interfaces are ubiquitous in natural environments, spanning atmospheric, geological, oceanographic, and biological systems, as well as in technical applications, such as fuel cells and membrane filtration. Where liquid water terminates at a surface, an interfacial region is formed, which exhibits distinct properties from the bulk aqueous phase. The unique properties of water are governed by the hydrogen-bonded network. The chemical and physical properties of the surface dictate the boundary conditions of the bulk hydrogen-bonded network and thus the interfacial properties of the water and any molecules in that region. Understanding the properties of interfacial water requires systematically characterizing the structure and dynamics of interfacial water as a function of the surface chemistry. In this review, we focus on the use of experimental surface-specific spectroscopic methods to understand the properties of interfacial water as a function of surface chemistry. Investigations of the air-water interface, as well as efforts in tuning the properties of the air-water interface by adding solutes or surfactants, are briefly discussed. Buried aqueous interfaces can be accessed with careful selection of spectroscopic technique and sample configuration, further expanding the range of chemical environments that can be probed, including solid inorganic materials, polymers, and water immiscible liquids. Solid substrates can be finely tuned by functionalization with self-assembled monolayers, polymers, or biomolecules. These variables provide a platform for systematically tuning the chemical nature of the interface and examining the resulting water structure. Finally, time-resolved methods to probe the dynamics of interfacial water are briefly summarized before discussing the current status and future directions in studying the structure and dynamics of interfacial water.