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Single atom (SA), noble metal catalysts are of interest due to high projected catalytic activity while minimizing cost. Common issues facing many synthesis methodologies include complicated processes, low yields of SA product, and production of mixtures of SA and nanoparticles (NPs). Herein we report a simple, room-temperature synthesis of single Pt-atom decorated, anatase Fe-doped TiO2 particles that leverages the Fe dopant as an engineered defect site to photodeposit and stabilize atomically dispersed Pt. Both particle morphology and Fe dopant location are based on thermodynamic principles (Gibbs-Wulff construction). CO-DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) reveals absence of bridge-bonded CO signal, confirming atomically dispersed Pt. XAS (X-ray absorption spectroscopy) of both Pt and Fe indicates Fe-O-Pt bonding that persists through catalytic cycling. Mass balance indicates that the Pt loading on single particles is 2.5 wt % Pt; the single Pt-atom decorated nanoparticle yield is 17%. Pt-containing particles show more than an order-of-magnitude increased photooxidation efficiency relative to particles containing only Fe. High single-atom-Pt yield, ease of synthesis, and high catalytic activity demonstrate the utility and promise of this method. The principles of this photodeposition synthesis allow for its generalizability toward other SA metals of catalytic interest.

Surface molecular structure is the arbiter in the contest between energy and entropy that largely determines how ice and snow crystals develop.

Physics and chemistry of ice surfaces are not only of fundamental interest but also have important impacts on biological and environmental processes. As ice surfaces—particularly the two prism faces—come under greater scrutiny, it is increasingly important to connect the macroscopic faces with the molecular-level structure. The microscopic structure of the ubiquitous ice Ih crystal is well-known. It consists of stacked layers of chair-form hexagonal rings referred to as molecular hexagons. Crystallographic unit cells can be assembled into a regular right hexagonal prism. The bases are labeled crystallographic hexagons. The two hexagons are rotated 30° with respect to each other. The linkage between the familiar macroscopic shape of hexagonal snowflakes and either hexagon is not obvious per se. This report presents experimental data directly connecting the macroscopic shape of ice crystals and the microscopic hexagons. Large ice single crystals were used to fabricate samples with the basal, primary prism, or secondary prism faces exposed at the surface. In each case, the same sample was used to capture both a macroscopic etch pit image and an electron backscatter diffraction (EBSD) orientation density function (ODF) plot. Direct comparison of the etch pit image and the ODF plot compellingly connects the macroscopic etch pit hexagonal profile to the crystallographic hexagon. The most stable face at the ice–water interface is the smallest area face at the ice–vapor interface. A model based on the molecular structure of the prism faces accounts for this switch.

Complex, soft interfaces abound in the environment, biological systems, and technological applications. Probing these interfaces, particularly those buried between two condensed phases presents many challenges. The only current method capable of probing such interfaces with molecular specificity is the vibrational spectroscopy, sum frequency generation (SFG). SFG is a nonlinear method, which often results both in small signals from minority species being lost in the noise and nonunique separation of resonances. Both issues can be addressed if the complex amplitude rather than the intensity—the square of the amplitude—spectrum is measured. Thus, several methods have been developed to determine the complex spectrum by measuring the sample of interest with respect to a nonresonant material. Incorrect assumptions about the phase of the nonresonant material can result in ambiguity about the sample complex components. This ambiguity can be removed if a phase standard is identified and the phase of the chosen reference material is measured against the standard. This paper reports both verification of a phase standard—Z-cut quartz—and measurement of the phase of gold against this standard. Using this phase standard, the standard phase of Au is determined to be −222° with 532 nm excitation and ppp polarization.

On the surface of water ice, a quasi-liquid layer (QLL) has been extensively reported at temperatures below its bulk melting point at 273 K. Approaching the bulk melting temperature from below, the thickness of the QLL is known to increase. To elucidate the precise temperature variation of the QLL, and its nature, we investigate the surface melting of hexagonal ice by combining noncontact, surface-specific vibrational sum frequency generation (SFG) spectroscopy and spectra calculated from molecular dynamics simulations. Using SFG, we probe the outermost water layers of distinct single crystalline ice faces at different temperatures. For the basal face, a stepwise, sudden weakening of the hydrogen-bonded structure of the outermost water layers occurs at 257 K. The spectral calculations from the molecular dynamics simulations reproduce the experimental findings; this allows us to interpret our experimental findings in terms of a stepwise change from one to two molten bilayers at the transition temperature.

Currently, the only techniques capable of delivering molecular-level data on buried or soft interfaces are the nonlinear spectroscopic methods: sum frequency generation (SFG) and second harmonic generation (SHG). Deducing molecular information from spectra requires measuring the complex components-the amplitude and the phase-of the surface response. A new interferometer has been developed to determine these components with orders-of-magnitude improvement in uncertainty compared with current methods. Both the sample and reference spectra are generated within the interferometer, hence the label nonlinear interferometer. The interferometer configuration provides experimenters with wide latitude for both the sample enclosure and reference material choice and is thus widely applicable. The instrument is described and applied to the well-studied octadecyltrichlorosilane (OTS) film. The OTS spectra support the interpretation that variation in fabrication solvent water content and substrate preparation account for differences in OTS spectra reported in the literature.

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.

Interaction between p-toluenesulfonic acid (pTSA) and water is studied at −20 °C in a CCl4 matrix. In CCl4 water exists as monomers with restricted rotational motion about its symmetry axis. Additionally, CCl4 is transparent in the hydrogen-bonded region; CCl4 thus constitutes an excellent ambient thermal energy matrix isolation medium for diagnosing interactions with water. Introducing pTSA-nH2O gives rise to two narrow resonances at 3642 cm–1 and at 2835 cm–1 plus a broad 3000–3550 cm–1 absorption. In addition, negative monomer symmetric and asymmetric stretch features relative to nominally dry CCl4 indicate that fewer water monomers exist in the cooled (−20 °C) acid solution than in room-temperature anhydrous CCl4. The negative peaks along with the broad absorption band indicate that water monomers are incorporated into clusters. The 3642 cm–1 resonance is assigned to the OH-π interaction with a cluster containing many water molecules per acid molecule. The 2835 cm–1 resonance is assigned to the (S-)O–...

Ice is a fundamental solid with important environmental, biological, geological, and extraterrestrial impact. The stable form of ice at atmospheric pressure is hexagonal ice, Ih. Despite its prevalence, Ihremains an enigmatic solid, in part due to challenges in preparing samples for fundamental studies. Surfaces of ice present even greater challenges. Recently developed methods for preparation of large single-crystal samples make it possible to reproducibly prepare any chosen face to address numerous fundamental questions. This review describes preparation methods along with results that firmly establish the connection between the macroscopic structure (observed in snowflakes, microcrystallites, or etch pits) and the molecular-level configuration (detected with X-ray or electron scattering techniques). Selected results of probing interactions at the ice surface, including growth from the melt, surface vibrations, and characterization of the quasi-liquid layer, are discussed.

Ice surfaces are used to assign five resonances in the hydrogen-bonded vibrational spectrum of water. Ice surface experiments are complemented with room temperature matrix isolation experiments providing compelling evidence that the OH vibration of the donor is affected by the environment around the lone pairs, i.e., compelling evidence for the importance of three-body interactions. In addition, longer-range, correlated motion produces a quadrupole at the ice surface.

Students are frequently presented with novel visualizations introducing scientific concepts and processes normally unobservable to the naked eye. Despite being unfamiliar, students are expected to understand and employ the visualizations to solve problems. Domain experts exhibit more competency than novices when using complex visualizations, but less is known about how and when learners develop representational fluency. This project examined students' moment-by-moment adoption patterns for scientific visualizations. In a laboratory experiment, introductory-level organic chemistry students viewed familiar ball-and-stick and novel electrostatic potential map representations while solving chemistry problems. Eye movement patterns, verbal explanations, and individual difference analyses showed that students initially relied on familiar representations, particularly for difficult questions. However, as the task unfolded, students with more prior knowledge began relying upon the novel visualizations. These results indicate adoption and fluent use of visualizations is not given; rather, it is a function of prior knowledge and unfolding experience with presented content.

This pilot study examined students' use of ball-and-stick images versus electrostatic potential maps when asked questions about electron density, positive charge, proton attack, and hydroxide attack with six different molecules (two alcohols, two carboxylic acids, and two hydroxycarboxylic acids). Students' viewing of these dual images was measured by monitoring eye fixations of the students while they read and answered questions. Results showed that students spent significantly more time with the ball-and-stick image when asked questions about proton or hydroxide attack, but equal time on the images when asked about electron density or positive charge. When comparing accuracy and time spent on the images, students who spent more time on the ball-and-stick when asked about positive charge were less likely to be correct, while those who spent more time with the potential map were more likely to be correct. The paper serves to introduce readers to eye-tracker data and calls for replication with a larger subject pool and for the inclusion of eye tracking as a chemical education research tool.

Unique among small molecules, water forms a nearly tetrahedral yet flexible hydrogen-bond network. In addition to its flexibility, this network is dynamic: bonds are formed or broken on a picosecond time scale. These unique features make probing the local structure of water challenging. Despite the challenges, there is intense interest in developing a picture of the local water structure due to water's fundamental importance in many fields of chemistry. Understanding changes in the local network structure of water near solutes likely holds the key to unlock problems from analyzing parameters that determine the three dimensional structure of proteins to modeling the fate of volatile materials released into the atmosphere. Pictures of the local structure of water are heavily influenced by what is known about the structure of ice. In hexagonal I(h) ice, the most stable form of solid water under ordinary conditions, water has an equal number of donor and acceptor bonds; a kind of symmetry. This symmetric tetrahedral coordination is only approximately preserved in the liquid. The most obvious manifestation of this altered tetrahedral bonding is the greater density in the liquid compared with the solid. Formation of an interface or addition of solutes further modifies the local bonding in water. Because the O-H stretching frequency is sensitive to the environment, vibrational spectroscopy provides an excellent probe for the hydrogen-bond structure in water. In this Account, we examine both local interactions between water and small solutes and longer range interactions at the aqueous surface. Locally, the results suggest that water is not a symmetric donor or acceptor, but rather has a propensity to act as an acceptor. In interactions with hydrocarbons, action is centered at the water oxygen. For soluble inorganic salts, interaction is greater with the cation than the anion. The vibrational spectrum of the surface of salt solutions is altered compared with that of neat water. Studies of local salt-water interactions suggest that the picture of the local water structure and the ion distribution at the surface deduced from the surface vibrational spectrum should encompass both ions of the salt.

A recently developed technique in sum frequency generation spectroscopy, polarization angle null (or PAN-SFG), is applied to two orientations of the prism face of hexagonal ice. It is found that the vibrational modes of the surface are similar in different faces. As in the basal face, the prism face of ice contains five dominant resonances: 3096, 3146, 3205, 3253, and 3386 cm(-1). On the basal face, the reddest resonance occurs at 3098 cm(-1); within the bandwidth, the same as the prism face. On both the prism and basal faces, this mode contains a significant quadrupole component and is assigned to the bilayer stitching hydrogen bonds. The bluest of the resonances, 3386 cm(-1), occurs slightly blue-shifted at 3393 cm(-1) in the basal face. The prism face has two orientations: one with the optic or c axis in the input plane (the plane formed by the surface normal and the interrogating beam propagation) and one with the c axis perpendicular to the input plane. The 3386 cm(-1) mode has significant intensity only with the c axis in the input plane. On the basis of these orientation characteristics, the 3386 cm(-1) mode is assigned to double-donor molecules in either the top half bilayer or in the lower half bilayer. On the basis of frequency considerations, it is assigned to double-donor molecules in the top half bilayer. These are water molecules containing a nonbonded lone pair. In addition to identification of the components of the broad hydrogen-bonded region, PAN-SFG measures the tangential vs longitudinal content of the vibrational modes. In accord with previous suggestions, the lower frequency modes are predominantly tangential, whereas the higher frequency modes are mainly longitudinal. On the prism face, the 3386 cm(-1) mode is entirely longitudinal.

Propane is one of several hydrocarbons known to form a clathrate hydrate. To probe interactions leading to clathrate nucleation, the water-propane interaction is investigated in carbon tetrachloride with infrared spectroscopy. Isotopic substitution provides compelling evidence that the water-propane interaction occurs between the propane methylene hydrogen atoms and the water lone pair. In addition, interaction between propane and water results in clustering of water molecules, a clustering that is identified by the appearance of a peak between the symmetric and asymmetric stretches of water.

A major challenge in hydrogen-bond research is interpreting the vibrational spectrum of water, arguably the most fundamental hydrogen bonding system. This challenge remains despite over a half century of progress in vibrational spectroscopy, largely due to a combination of the huge oscillator strength and the enormous width of the hydrogen-bond region. Lack of assignment of the resonances in the hydrogen-bond region hinders investigation of interactions between water and solutes. This lack-of-interpretation issue is an even more significant problem for studies of the aqueous interface. Numerous solutes are known to have an effect, some very dramatic, on the shape of the surface spectrum. These effects, however, are but tantalizing teasers because lack of interpretation means that the changes cannot be used to diagnose the effect of solutes or impinging gas-phase molecules on the surface. In the reported work two techniques are used to probe the origin of vibrational resonances in the H-bonded region: the surface sensitive technique sum frequency generation (SFG) and room-temperature matrix isolation spectroscopy (RT-MIS). A polarization technique called polarization angle null (PAN) has been developed that extends SFG and enables identification of resonances. The result of applying PAN–SFG to single crystal, I h ice is identification of at least nine underlying resonances and assignment of two of these. One resonance is correlated with the crystal temperature and is a sensitive probe for interactions that disrupt long range order on the surface – it is a morphology reporter. The second is associated with weakly bonded, double-donor water molecules. This resonance is sensitive to interaction of hydrogen bond donors, i.e. acids, with the surface. Both modes are more correctly pictured as collective modes. These two assignments are the first definitive assignments in the hydrogen-bond region for the aqueous surface . The effect of salts on the vibrational spectrum of water is also probed with a recently developed room-temperature, matrix-isolation technique. The matrix environment demonstrates that salts containing large anions with small cations support water–water hydrogen bonds with a vibrational resonance that has similar characteristics as the SFG spectra of salt solutions.

It is generally expected that ions in an aqueous ionic solution in contact with a hydrophobic phase enter the hydrophobic phase accompanied by a hydration shell. This expectation suggests that the ion mole fraction in the hydrophobic phase is less than, or at most, equal to that of water. Both gravimetric and spectroscopic evidence shows that for a model hydrophobic phase, carbon tetrachloride, this is not the case: In contact with a 1 M simple salt solution (sodium or potassium halide), the salt concentration in carbon tetrachloride ranges from 1.4 to nearly 3 times that of water. Infrared spectra of the OH stretch region support a model in which water associates with the cation, primarily as water monomers. Salts containing larger, more polarizable anions can form outer-sphere ion pairs that support water dimers, giving rise to a spectral signature at 3440 cm(-1). In CCl(4), the infrared spectral signature of the normally strongly ionized acid HCl clearly shows the presence of molecular HCl. Additionally, the presence of a Q branch for HCl indicates restricted rotational motion. The spectral and gravimetric data provide compelling evidence for ion clusters in the hydrophobic phase, which is a result that may have implications for hydrophobic matter in both biological and environmental systems.

The Jahn-Teller effect for strong coupling between a triply degenerate electronic state and a triply degenerate vibrational mode (T × t) has been treated using a unitary transformation method. With this method it was possible to diagonalize the major portion of the interaction and therefore possible to calculate approximate energy levels for moderate to strong coupling. For moderate coupling good agreement was found between our analytical calculation and the numerical calculations (available only for weak to moderate coupling).

In this work we present the vibrational spectrum of a hydrogen-bonded complex, nitric acid–water, in the region 6000–11 000 cm −1 using Fourier-transform infrared spectroscopy. This study builds on previous observation of the nitric acid–water complex in the mid-infrared via a carbon tetrachloride matrix to investigate the vibrational overtone region of the complex at ambient conditions. Our experimental observations of ν OH = 1, 2 and 3 of the nitric acid–water complex provides a testing ground for the simple harmonically coupled anharmonic oscillator local-mode model treatment of OH-vibrational overtone spectra of hydrogen-bonded complexes.

The physics of the ice crystal surface and its interaction with adsorbates are not only of fundamental interest but also of considerable importance to terrestrial and planetary chemistry. Yet the atomic-level structure of even the pristine ice surface at low temperature is still far from well understood. This computational study focuses on the pattern of dangling H and dangling O (lone pairs) atoms at the basal ice surface. Dangling atoms serve as binding sites for adsorbates capable of hydrogen- and electrostatic bonding. Extension of the well known orientational disorder (“proton disorder”) of bulk crystal ice to the surface would naturally suggest a disordered dangling atom pattern; however, extensive computer simulations employing two different empirical potentials indicate significant free energy preference for a striped phase with alternating rows of dangling H and dangling O atoms, as suggested long ago by Fletcher [Fletcher NH (1992) Philos Mag 66:109–115]. The presence of striped phase domains within the basal surface is consistent with the hitherto unexplained minor fractional peaks in the helium diffraction pattern observed 10 years ago. Compared with the disordered model, the striped model yields improved agreement between computations and experimental ppp -polarized sum frequency generation spectra.

Infrared spectroscopy has been used to probe the interaction between water and the hydrophobic solvent, carbon tetrachloride. At room temperature, water exists as monomers in carbon tetrachloride, presenting a system for studying the rotational properties of water free of strong hydrogen-bonding. The rotational structure suggests a very anisotropic motion consisting of essentially free rotation about the symmetry axis and highly hindered rotation about the two perpendicular axes of the asymmetric water molecule. The rotational lifetime is significantly shortened relative to gas-phase water. An upper limit of 0.93 ps is deduced from the spectrum. Interaction with carbon tetrachloride also slightly enhances the intensity of the symmetric stretch. The results are compared with results of interactions between water and the cations Li+, Na+, K+, and Cs+. It is concluded that the attractive interaction is between the oxygen of water and the electropositive carbon of carbon tetrachloride.

Sum frequency generation (SFG) vibrational spectroscopy along with the molecular probe, methanol, has been used to investigate the surface chemistry of TiO2. Methanol on TiO2 has two adsorption modes: a molecular physisorption mode and a dissociative chemisorption mode. Doping TiO2 with iron leads to quenching of the dissociative adsorption. Electron paramagnetic resonance (EPR) spectra of iron-doped TiO2 particles (particle size > 100 nm) compared to the same particles leached with water show that at least some of the iron is located on the surface. Particles are also characterized with UV–visible spectroscopy.

The interaction between nitric acid and water is investigated under conditions of limited water activity: at the aqueous interface and dispersed in the hydrophobic solvent, carbon tetrachloride. It is found that water forms only monomers in carbon tetrachloride at temperatures >0 °C. Addition of nitric acid shows features due to molecular nitric acid and the hydrates: nitric acid monohydrate (NAM), nitric acid dihydrate (NAD), and nitric acid trihydrate (NAT). Spectral features from water molecules weakly bound to nitric acid are slightly red-shifted from the symmetric and antisymmetric stretch of water monomers. The solvent-isolated results support a model in which nitric acid is molecular at the aqueous−nitric acid solution interface. The presence of molecular nitric acid at the interface has important consequences for atmospheric chemistry.

The ability to prepare single-crystal faces has become central to developing and testing models for chemistry at interfaces, spectacularly demonstrated by heterogeneous catalysis and nanoscience. This ability has been hampered for hexagonal ice, Ih--a fundamental hydrogen-bonded surface--due to two characteristics of ice: ice does not readily cleave along a crystal lattice plane and properties of ice grown on a substrate can differ significantly from those of neat ice. This work describes laboratory-based methods both to determine the Ih crystal lattice orientation relative to a surface and to use that orientation to prepare any desired face. The work builds on previous results attaining nearly 100% yield of high-quality, single-crystal boules. With these methods, researchers can prepare authentic, single-crystal ice surfaces for numerous studies including uptake measurements, surface reactivity, and catalytic activity of this ubiquitous, fundamental solid.

Sum frequency generation (SFG) vibrational spectroscopy is used to investigate the surface adsorption of three probe molecules-acetic acid, methanol, and water--on a film composed of nanoscale anatase TiO(2) particles. On the TiO(2) surface, only one adsorption mode, chemisorption, is observed for acetic acid. This is evidenced by one sharp SFG peak in the C-H region, which is stable with time and robust both to evacuation and to the addition of water. A Langmuir constant of (9.21 +/- 0.71) x 10(3) is determined from the adsorption isotherm. In the case of methanol adsorption, however, there are two adsorption modes, molecular physisorption and dissociative chemisorption. The corresponding SFG signals are stable with time but diminished with addition of water. Changes in the SFG features for methanol and for the methoxy species with addition of water and subsequent evacuation provide the first experimental proof of reversible hydroxylation and dehydroxylation at the TiO(2) surface. For water adsorption, only one mode, physisorption, is observed on the hydroxylated TiO(2) surface. The water adlayer is mobile, as is evidenced by variation of the water H-bonded SFG signal with time. Competitive adsorption among the three molecular probes is clearly resolved by in situ SFG measurements. The adsorption strength follows the order acetic acid (strongest), methanol, water (weakest). The adsorption order as well as the difference in response of methanol versus acetic acid adsorption to addition of water has direct implications for understanding TiO(2) photocatalysis as well as the surface modifications involved in TiO(2) photoelectrochemical solar cells and processes in TiO(2) nanomaterial synthesis and assembly.

Knowledge of hydroxide ion’s spectral signature is an essential ingredient for finding it at the water surface. In solutions, the intense hydrogen-bonded resonance of water often hinders unambiguous characterization of the OH − stretching frequency and linewidth. These parameters are likely to be modified at the hydrophobic water–vapor interface. This communication reports the first experimental observation of hydroxide ion that is free from interaction with bulk water. OH − is isolated in a hydrophobic environment and exhibits a sharp resonance at 3634 cm −1 , as confirmed by isotopic substitution. The feature is red shifted from that of dangling OH bonds by 29 cm −1 .

Many interfacial studies on solid surfaces, for example, quartz/water, assume that a standard cleaning procedure regenerates the surface reproducibly. In the reported work, the results of two surface specific techniques, sum frequency generation (SFG) spectroscopy and atomic force microscopy, show that the effects of prolonged exposure to Nanopure water and to pH 10 NaOH are distinctly different. In conjunction with the experimental data, molecular mechanics is used to correlate the SFG spectral frequencies to the hydrogen stretching vibrations of the surface-bound water molecules. It is found that after 17 days of soaking in water, water molecules penetrate into the SiO2 matrix to produce a swollen and amorphous layer; it is likely that broken Si-O bonds from the polishing process serve as nucleation sites for hydration and swelling. Disorder introduced in the interfacial water layer is detected by the rising intensity of the weakly hydrogen-bonded SFG peak at 3450 cm(-1). Dominance of the 3450 cm(-1) is absent in a pH 10, NaOH-soaked quartz disk, indicating that the strong hydrogen-bonded network in water remains intact.

Sum frequency generation (SFG) vibrational spectroscopy is applied to study the surface characteristics of a film composed of nanoscale TiO2 particles, in which methanol is employed as a molecular probe. The SFG signal from methanol adsorbed on the TiO2 film surface in the CH region consists of four primary peaks located at 2828, 2855, 2935, and 2968 cm-1. The peaks at 2855 and 2968 cm-1 arise from molecular methanol; the other two are attributed to methoxy produced by dissociative chemisorption of methanol on TiO2. Changes in the SFG features with methanol pressure and with surface temperature provide the first direct proof of methoxy generation on nanoparticulate films. In comparison with molecular methanol, methoxy sits more perpendicularly on the surface. The chemisorption is barrierless, and a Langmuir constant of (2.13 ± 0.14) × 103 is derived from the adsorption isotherm. Moreover, UV pretreatment of the TiO2 film substantially increases the methoxy signal. Changes in SFG intensity are in good agre...

Visible-infrared sum frequency generation (SFG) has been applied to probe molecular species on the surface of nanoparticulate anatase TiO2 films. A trace hydrocarbon film on the surface, not detected by Fourier transform infrared spectroscopy (FTIR), is easily sensed with SFG. The first direct observation of hydroxyl groups (-OH) on the TiO2 film surface by SFG is reported. A broad vibrational band with multiple peaks in the region of 3500−3800 cm-1 ascribed to OH groups reflects the surface heterogeneity of the material.

Sum frequency generation (SFG) spectroscopy is a unique tool for probing the vibrational structure of numerous interfaces. Since SFG is a nonlinear spectroscopy, it has long been recognized that measuring only the intensity-the absolute square of the surface response-limits the potential of SFG for examining interfacial interactions and dynamics. The potential is unlocked by measuring the phase-sensitive or imaginary response. As with any phase, the phase-sensitive SFG response is measured relative to a reference; the spatial relationship between the phase reference and the sample modulates the observed interference intensity and impacts sensitivity and accuracy. We have designed and implemented a nonlinear interferometer to directly measure the phase-sensitive response. If the phase of the reference is known, then the interferometer produces an absolute phase of the surface. Compared to current configurations, phase accuracy and stability are greatly improved due to active stabilization of the sample-reference position. The design is versatile and thus can be used for any system that can be probed with SFG including buried interfaces and those with high vapor pressure. Feasibility and advantages of the interferometer are demonstrated using an octadecyltrichlorosilane film on fused silica.

Tetrahydrofuran (THF) is well-known as a clathrate former as well as a promoter for gas hydrate formation. This work examines interactions between water and tetrahydrofuran via the effect on water's vibrational spectrum. Due to water's large oscillator strength in the hydrogen-bonded region, interactions are diagnosed by isolating small clusters in a transparent medium (carbon tetrachloride in this study). A weak THF/water hydrogen bond is reflected by a 3450 cm(-1) OH-donor vibration (blue shifted from the water/water hydrogen bond) and a 3685 cm(-1) nonbonded OH stretch (blue shifted 22 cm(-1) from the decoupled OH stretch in this medium). Increasing the THF concentration results in another 20 cm(-1) blue shift of the OH-donor stretch. Additional THF does not complex with free water but rather joins with existing THF/water structures to form a cluster enriched in THF. These results complement previous work examining THF vibrations in clathrate hydrates. Together, they generate a picture in which water mediates between THF pairs--mediation that affects vibrational frequencies of both species. In addition to a frequency shift, water's hydrogen-bonded resonance gains oscillator strength due to its mediating configuration.

The Raman spectra of aqueous ammonia solutions have been obtained between -40 and 25 degrees C. The Raman spectrum of neat water was also obtained at 25 degrees C and is characterized by two broad peaks observed at 3200 and 3400 cm(-1). The spectrum due to water is subtracted to determine the NH(3) spectrum at all temperatures. In ammonia-water solutions, the spectrum shows three features at measured displacements of 3250, 3316, and 3400 cm(-1). The feature at 3316 cm(-1) is assigned to the Q branch of the symmetric stretch. The broad, weak features at 3250 and 3400 cm(-1), previously assigned to rotational bands, are assigned to combination bands. The NH(3) combination bands are assigned by comparing with sum frequency generation (SFG) experiments, monitoring changes with temperature, and analyzing the polarization data. The rotational structure of the Q band is also discussed. As the temperature is lowered from 25 to -40 degrees C, an increase in the Raman intensity is observed for all bands. The relative Raman scattering cross section is determined from the numerically integrated area of the NH(3) Q branch at each temperature. Copyright 2001 Academic Press.

Ammonia–water complexes have been detected with sum frequency generation (SFG) at the liquid/vapor interface of concentrated ammonia solutions (0.3x NH3, x=bulk mole fraction). SFG spectra taken with the ssp polarization combination (s-polarized sum frequency signal, s-polarized visible light, p-polarized infrared beam) are dominated by the N–H symmetric stretch (ν1) at 3312 cm−1 and a weaker deformation mode (2ν4) at 3200 cm−1. The dangling (free) OH peak due to water at 3700 cm−1 is suppressed at this concentration, indicating that water molecules are complexed through hydrogen bonds to ammonia at the interface. The polarization dependence of ν1 indicates that the NH3 C3 molecular axis is tilted with respect to the surface normal by θ⩽38°. Variation of the antisymmetric stretch, ν3, sum frequency signal intensity with polarization restricts the lower limit for surface ammonia tilt angles such that 25°⩽θ⩽38°. Based on this orientation, the structure of the ammonia–water complex is discussed.

The liquid interface of aqueous solutions is of central importance to numerous phenomena from cloud processing of combustion generated oxides to corrosion degradation of structural materials to transport across cell membranes. Despite the importance of this interface, little molecular-level information was known about it prior to the last decade-and-a-half. Molecular-level information is important not only for a fundamental understanding of processes at interfaces, but also for predicting methods for diminishing deleterious effects. Recently, the non-linear spectroscopic method, sum frequency generation (SFG), has been applied to the investigation of the structure of the liquid interface. This review focuses on the liquid-air interface of aqueous solutions containing soluble, ionic species - H2SO4, HNO3, HCl, alkali sulphates and bisulphates, NaCl and NaNO3 - as well as soluble molecular species-glycerol, sulphuric acid and ammonia. Ionic materials influence the structure of water at the interface through...

The vapor−liquid interface of aqueous inorganic acid and salt solutions are examined using sum frequency generation (SFG). The results show that the SFG intensity of hydrogen-bonded water on 0.01x acid (HCl, HNO3, and H2SO4) solutions, where x = mole fraction, is greatly enhanced compared to the 0.01x solution of the corresponding salts (NaCl, NaNO3, and KHSO4). This suggests that either surface water molecules on the acid solutions orient with the dipole in a more vertically aligned fashion than those on the salt solutions or that more layers of water are ordered. These results are interpreted with an electric double layer model, in which the double layer is composed of subsurface anions and cations. The weak association of protons, as opposed to Na+ or K+, with anions results in a greater electric field at the surface on acid solutions. The perturbation of surface water on ionic solutions is discussed in terms of electrostatic and displacement mechanisms.

Sum frequency generation spectra indicate that the surface water of liquid sulfuric acid solutions varies as a function of concentration, but not with temperature. At 0.1 x H 2 SO 4 (where x =mole fraction, 38 wt%), subsurface ionic species orient surface water molecules. The surface of 0.2 x H 2 SO 4 (58 wt%) solutions, however, features H 2 SO 4 /H 2 O complexes both at 273 K and supercooled at 216 K. The results support a picture of stratospheric chemistry in which sulfuric acid aerosols are coated with hydrogen-bonded water/sulfuric acid complexes.

The surfaces of aqueous HNO3 solutions and liquid HNO3 are examined using sum frequency generation (SFG). A molecular-level picture of these atmospherically relevant systems is developed. Consisten...

The surface-specific vibrational spectroscopy, sum frequency generation, has been used to study the structure of water at air/liquid interfaces of inorganic, ionic solutions. The experiments suggest that ions in solution reorient surface water into a more structured, hydrogen-bonded network compared with the neat water/air interface. In more concentrated solutions, there is a high degree of ion association. A model based on Hard Soft Acid Base theory indicates that associated ions are able to penetrate closer to the air interface than unassociated ions. Associated ions incorporate surface water into the ion-pair hydration shell.

The influence of dissolved HCl on the surface of water has been investigated with sum frequency generation (SFG) spectroscopy. Ions in solution cause water on the surface to be reoriented relative to pure water, with hydrogen atoms directed toward the bulk solution. There is no signal due to molecular HCl suggesting that oriented water molecules, not molecular HCl, dominate the surface. A model is proposed to account for the reported SFG results as well as surface tension and surface potential measurements. The result suggests that application of the Gibbs equation to determine surface excess may need to be reevaluated.

Sum frequency generation (SFG) is used to obtain the vibrational spectrum of aqueous ammonia solution. The SF spectrum of concentrated solutions is dominated by the N–H symmetric stretch (ν1) at 3312 cm−1 and a weaker deformation mode (2ν4) at 3200 cm−1. The free OH peak of water is suppressed, compared to pure water, and the hydrogen-bonded region (3000–3450 cm−1) has interference in the presence of ammonia molecules at the interface. In dilute solution, ν1 is less intense and the free OH peak of water appears. These observations confirm the existence of an ammonia–water complex at the liquid/vapor interface.

Sum frequency generation spectroscopy has been used to obtain the vibrational spectrum of HCl on the surface of a liquid. HCl was studied on the surface of 96 wt % H2SO4, 12 M HCl solution, liquid HCl and glass, of which only liquid HCl produces a resonant signal. Implications for the form of HCl on surfaces and the reactions in the atmosphere are discussed.

The vibrational structure of water at the air/solution interface of an ionic solution has been obtained for the first time. Using vibrational sum frequency generation it is determined that ions in solution have a large orientational effect on the structure of the surface water. Electrolytic solutions, ionic in nature, cause water to be oriented into a more regular hydrogen-bonded network through an electric double layer at the interface. In electrolytic solutions where molecular or associated H 2 SO 4 or Cs 2 SO 4 species dominate, the surface water molecules are bound into hydrate complexes. These effects are explained using hard soft acid base (HSAB) theory.

Structural data about water at the air/water interface of sulfuric acid solutions have been obtained with sum frequency generation spectroscopy. Sulfuric acid significantly affects the orientation of water at the interface. With as little as 0.01x (mole fraction) sulfuric acid, water is more highly oriented compared with the pure water surface. Surface water with one hydrogen free of hydrogen bonding, a “free OH”, decreases in concentration as the sulfuric acid concentration is increased. Finally, there are no sulfuric acid free OH groups projecting out of the surface for solutions from 0.01x to 0.9x sulfuric acid. Observations of the structure of water are consistent with the fact that sulfuric acid/water solutions change from ionic in nature at low concentrations ( 0.4x).

The OH and CH regions of glycerol/water mixtures in the concentration range 0.0−1.0 mole fraction have been investigated with vibrational sum frequency generation. Glycerol is found to partition to the surface of these solutions in all concentrations. Neat glycerol surfaces contain no free OH groups projecting into the vapor. The surface orientation of glycerol is constant through most of the concentration range.

The prism face of single crystal ice I(h) has been studied using sum frequency vibrational spectroscopy focusing on identification of resonances in the hydrogen-bonded region. Several modes have been observed at about 3400 cm(-1); each mode is both polarization and orientation dependent. The polarization capabilities of sum frequency generation (SFG) are used in conjunction with the crystal orientation to characterize three vibrational modes. These modes are assigned to three-coordinated water molecules in the top-half bilayer having different bonding and orientation motifs.