Measuring complex SFG: Characterizing a phase reference.

Reactions and interactions at interfaces play pivotal roles in processes ranging from atmospheric aerosols influencing climate to battery electrodes determining charge-discharge rates to defects in catalysts controlling the fate of reactants to the outcome of biological processes at membrane interfaces. Tools to probe these surfaces at the atomic-molecular level are thus critical. Chief among non-invasive probes is the vibrational spectroscopy sum frequency generation (SFG).

Choose Your Dopant─Ultrananotitania Directing Photocatalytic Electron Destination.

Anatase titania photocatalysts are the subject of thousands of papers annually. Since the photoefficiency is low, doping TiO photocatalysts with various elements is a widely employed technique to enhance performance. However, studies involving different catalyst sizes, morphologies, or dopants often yield varied or even contradictory conclusions.

High phase resolution: Probing interactions in complex interfaces with sum frequency generation.

An often-quoted statement attributed to Wolfgang Pauli is that God made the bulk, but the surface was invented by the devil. Although humorous, the statement really reflects frustration in developing a detailed picture of a surface. In the last several decades, that frustration has begun to abate with numerous techniques providing clues to interactions and reactions at surfaces.

Photosynthesis of a Photocatalyst: Single Atom Platinum Captured and Stabilized by an Iron(III) Engineered Defect.

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 TiO particles that leverages the Fe dopant as an engineered defect site to photodeposit and stabilize atomically dispersed Pt.

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, 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.

Gold as a standard phase reference in complex sum frequency generation measurements.

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).

Matrix Isolation Spectroscopy: Aqueous p-Toluenesulfonic Acid Solvation.

Interaction between p-toluenesulfonic acid (pTSA) and water is studied at -20 °C in a CCl matrix. In CCl water exists as monomers with restricted rotational motion about its symmetry axis. Additionally, CCl is transparent in the hydrogen-bonded region; CCl thus constitutes an excellent ambient thermal energy matrix isolation medium for diagnosing interactions with water.

Nonlinear interferometer: Design, implementation, and phase-sensitive sum frequency measurement.

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.

Single-crystal ice surfaces unveil connection between macroscopic and molecular structure.

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 crystal is well-known.

Ice Surfaces.

Ice is a fundamental solid with important environmental, biological, geological, and extraterrestrial impact. The stable form of ice at atmospheric pressure is hexagonal ice, I. Despite its prevalence, I remains an enigmatic solid, in part due to challenges in preparing samples for fundamental studies.

Measuring Complex Sum Frequency Spectra with a Nonlinear Interferometer.

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.

High yield, single crystal ice via the Bridgman method.

The surface chemistry of ice and of water is an important topic of study, especially given the role of ice and water in shaping the environment. Although snow, granular, and polycrystalline ice are often used in research, there are applications where large surface areas of a known crystallographic plane are required. For example, fundamental spectroscopy or scattering studies rely on large area samples of known crystalline orientation.

Producing desired ice faces.

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.

Hydrogen Bonding between Water and Tetrahydrofuran Relevant to Clathrate Formation.

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).

Insights into hydrogen bonding via ice interfaces and isolated water.

Water in a confined environment has a combination of fewer available configurations and restricted mobility. Both affect the spectroscopic signature. In this work, the spectroscopic signature of water in confined environments is discussed in the context of competing models for condensed water: (1) as a system of intramolecular coupled molecules or (2) as a network with intermolecular dipole-dipole coupled O-H stretches.

Best face forward: crystal-face competition at the ice-water interface.

The ice-water interface plays an important role in determining the outcome of both biological and environmental processes. Under ambient pressure, the most stable form of ice is hexagonal ice (Ih). Experimentally probing the surface free energy between each of the major faces of Ih ice and the liquid is both experimentally and theoretically challenging.

Hydrogen bonding in the prism face of ice I(h) via sum frequency vibrational spectroscopy.

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.

When do spatial abilities support student comprehension of STEM visualizations?

Spatial visualization abilities are positively related to performance on science, technology, engineering, and math tasks, but this relationship is influenced by task demands and learner strategies. In two studies, we illustrate these interactions by demonstrating situations in which greater spatial ability leads to problematic performance. In Study 1, chemistry students observed and explained sets of simultaneously presented displays depicting chemical phenomena at macroscopic and particulate levels of representation.

Differing photo-oxidation mechanisms: electron transfer in TiO2 versus iron-doped TiO2.

Low-level iron doping has been found to alter the photo-oxidation mechanism of TiO(2) by efficiently activating molecular oxygen. In the absence of iron, TiO(2) either reduces water or stores the electron. Quantitatively, 0.

Water: a responsive small molecule.

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.

Competitive binding of methanol and propane for water via matrix-isolation spectroscopy: implications for inhibition of clathrate nucleation.

Methanol is a well-known thermodynamic inhibitor of clathrate hydrate formation. The interactions responsible for the inhibition, however, are not well-identified. Propane is a relatively simple hydrocarbon that forms a clathrate hydrate under mild conditions.

Hydrogen bonding in the hexagonal ice surface.

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).

Multiplexed polarization spectroscopy: measuring surface hyperpolarizability orientation.

Infrared-visible sum frequency generation (SFG) has seen increasing usage as a surface probe, particularly for liquid interfaces since they are amenable to few alternate probes. Interpreting the SFG data to arrive at a molecular-level configuration on the surface, however, remains a challenge. This paper reports a technique for analyzing and interpreting SFG data--called polarization-angle null or PAN-SFG.