Heterodyne-detected electronic sum frequency generation: “Up” versus “down” alignment of interfacial molecules

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.

Ultrafast Formation of Charge Transfer Trions at Molecular-Functionalized 2D MoS2 Interfaces

In this work, we investigate trion dynamics occurring at the heterojunction between organometallic molecules and a monolayer transition metal dichalcogenide (TMD) with transient electronic sum frequency generation (tr-ESFG) spectroscopy. By pumping at 2.4 eV with laser pulses, we have observed an ultrafast hole transfer, succeeded by the emergence of charge-transfer trions. This observation is facilitated by the cancellation of ground state bleach and stimulated emission signals due to their opposite phases, making tr-ESFG especially sensitive to the trion formation dynamics. The presence of charge-transfer trion at molecular functionalized TMD monolayers suggests the potential for engineering the local electronic structures and dynamics of specific locations on TMDs and offers a potential for transferring unique electronic attributes of TMD to the molecular layers.

Noncollinear Optical Parametric Amplification of Broadband Infrared Sum Frequency Generation Vibrational Spectroscopy

Sum-frequency generation (SFG) vibrational spectroscopy is an invaluable tool in surface science, known for its specificity to surfaces and interfaces. Despite its wide application, it is often hampered by weak signal detection. Here, we present an innovative enhancement technique of postsample amplification, using a picosecond noncollinear optical parametric amplifier (NOPA). We conducted a systematical investigation into the impact of different intensities of pump and SFG seed light, as the input signal in NOPA, and demonstrated this method on the octadecanethiol (ODT) molecules on gold films. The amplified SFG by NOPA reproduced the SFG vibrational spectra, enhanced by about 4 orders of magnitude but with broader spectral resolution due to the short pulse width of the pump light in NOPA. This study makes it possible to realize highly sensitive SFG measurements, marking a significant advancement in spectroscopic analysis techniques.

Aqueous chemimemristor based on proton-permeable graphene membranes

Memristive devices, electrical elements whose resistance depends on the history of applied electrical signals, are leading candidates for future data storage and neuromorphic computing. Memristive devices typically rely on solid-state technology, while aqueous memristive devices are crucial for biology-related applications such as next-generation brain-machine interfaces. Here, we report a simple graphene-based aqueous memristive device with long-term and tunable memory regulated by reversible voltage-induced interfacial acid-base equilibria enabled by selective proton permeation through the graphene. Surface-specific vibrational spectroscopy verifies that the memory of the graphene resistivity arises from the hysteretic proton permeation through the graphene, apparent from the reorganization of interfacial water at the graphene/water interface. The proton permeation alters the surface charge density on the CaF2 substrate of the graphene, affecting graphene's electron mobility, and giving rise to synapse-like resistivity dynamics. The results pave the way for developing experimentally straightforward and conceptually simple aqueous electrolyte-based neuromorphic iontronics using two-dimensional (2D) materials.

Substrate effect on charging of electrified graphene/water interfaces

Graphene, a transparent two-dimensional (2D) conductive electrode, has brought extensive new perspectives and prospects to electrochemical systems, such as chemical sensors, energy storage, and energy conversion devices. In many of these applications, graphene, supported on a substrate, is in contact with an aqueous solution. An increasing number of studies indicate that the substrate, rather than graphene, determines the organization of water in contact with graphene, i.e., the electric double layer (EDL) structure near the electrified graphene, and the wetting behavior of the graphene: the graphene sheet is transparent in terms of its supporting substrate. By applying surface-specific heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy to the silicon dioxide (SiO2)-supported graphene electrode/aqueous electrolyte interface and comparing the data with those for the calcium fluoride (CaF2)-supported graphene [Y. Wang et al., Angew. Chem., Int. Ed., 2023, 62, e202216604], we discuss the impact of the different substrates on the charging of both the graphene and the substrate upon applying potentials. The SiO2-supported graphene shows pseudocapacitive behavior, consistent with the CaF2-supported graphene case, although the surface charges on SiO2 and CaF2 differ substantially. The SiO2 surface is already negatively charged at +0.57 V (vs. Pd/H2), and the negative surface charge is doubled when negative potentials are applied, in contrast with the CaF2 case, where the positive charge is reduced when negative potentials are applied. Interestingly, the charging of the graphene sheet is almost identical between the negatively charged SiO2 surface and positively charged CaF2 surface, demonstrating that the graphene charging is decoupled from the charging of the substrates.

Absolute local conformation of poly(methyl methacrylate) chains adsorbed on a quartz surface

Polymer chains at a buried interface with an inorganic solid play a critical role in the performance of polymer nanocomposites and adhesives. Sum frequency generation (SFG) vibrational spectroscopy with a sub-nanometer depth resolution provides valuable information regarding the orientation angle of functional groups at interfaces. However, in the case of conventional SFG, since the signal intensity is proportional to the square of the second-order nonlinear optical susceptibility and thereby loses phase information, it cannot be unambiguously determined whether the functional groups face upward or downward. This problem can be solved by phase-sensitive SFG (ps-SFG). We here applied ps-SFG to poly(methyl methacrylate) (PMMA) chains in direct contact with a quartz surface, shedding light on the local conformation of chains adsorbed onto the solid surface. The measurements made it possible to determine the absolute orientation of the ester methyl groups of PMMA, which were oriented toward the quartz interface. Combining ps-SFG with all-atomistic molecular dynamics simulation, the distribution of the local conformation and the driving force are also discussed.

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.

Ice Recrystallization Inhibition Activity of Silk Proteins

The cryopreservation of cells, tissue, and organs is essential in both fundamental research and practical applications, such as modern regenerative medicine and technological applications. However, the formation of ice crystals during ice recrystallization can have harmful or even fatal effects on biological systems. To address this challenge, we explore the ice recrystallization inhibition (IRI) activity of two natural silk proteins of Bombyx mori, fibroin and sericin. We found that silk fibroin (SF) had higher ice recrystallization inhibition activity than silk sericin (SS). Moreover, SF aqueous solutions perform better in inhibiting ice recrystallization than SF phosphate-buffered saline solutions. Sum-frequency generation spectroscopy shows that stronger electrostatic interactions are responsible for the higher IRI ability of SF. This work is significant for broadening the applications of silk proteins in biomedical fields.

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. Often these techniques require considerable prior knowledge. Complex mixtures on irregular or soft surfaces-complex interfaces-thus represent the last frontier. Two optical techniques: sum frequency generation (SFG) and second harmonic generation (SHG) are beginning to lift the veil on complex interfaces. Of these techniques, SFG with one excitation in the infrared has the potential to provide exquisite molecular- and moiety-specific vibrational data. This Perspective is intended both to aid newcomers in gaining traction in this field and to demonstrate the impact of high-phase resolution. It starts with a basic description of light-induced surface polarization that is at the heart of SFG. The sum frequency is generated when the input fields are sufficiently intense that the interaction is nonlinear. This nonlinearity represents a challenge for disentangling data to reveal the molecular-level picture. Three, high-phase-resolution methods that reveal interactions at the surface are described.

Theoretical study of electronic sum frequency generation spectroscopy to assess the buried interfaces

This article provides a comprehensive theoretical background of electronic sum frequency generation (ESFG), a second-order nonlinear spectroscopy technique. ESFG is utilized to investigate both exposed and buried interfaces, which are challenging to study using conventional spectroscopic methods. By overlapping two incident beams at the interface, ESFG generates a beam at the sum of their frequencies, allowing for the extraction of valuable interfacial molecular information such as molecular orientation and density of states present at interfaces. The unique surface selectivity of ESFG arises from the absence of inversion symmetry at the interfaces. However, detecting weak signals from interfaces requires the ultrafast lasers to generate a sufficiently strong signal. By understanding the theoretical foundations of ESFG presented in this article, readers can gain a solid grasp of the basics of ESFG spectroscopy.

Development of electronic sum frequency generation spectrophotometer to assess the buried interfaces

The interfacial region between two bulk media in organic semiconductor based devices, such as organic field-effect transistors (OFETs), organic light-emitting diodes, and organic photovoltaics, refers to the region where two different materials such as an organic material and an electrode come in contact with each other. Although the interfacial region contains a significantly smaller fraction of molecules compared to the bulk, it is the primary site where many photoinduced excited state processes occur, such as charge transfer, charge recombination, separation, energy transfer processes, etc. All such photoinduced processes have a dependence on molecular orientation and density of states at the interfaces, therefore having an understanding of the interfacial region is essential. However, conventional spectroscopic techniques, such as surface-enhanced Raman scattering, x-ray photoelectron spectroscopy, atomic force microscopy, etc., face limitations in probing the orientation and density of states of interfacial molecules. Therefore, there is a need for noninvasive techniques capable of efficiently investigating the interfaces. The electronic sum frequency generation (ESFG) technique offers an interface selectivity based on the principle that the second-order nonlinear susceptibility tensor, within the electric dipole approximation, is zero in the isotropic bulk but nonzero at interfaces. This selectivity makes ESFG a promising spectroscopy tool to probe the molecular orientation and density of states at the buried interface. For beginners interested in employing ESFG to study the density of states at the interface, a detailed description of the experimental setup is provided here.

Local pH at Nonionic and Zwitterionic Lipid/Water Interfaces Revealed by Heterodyne-Detected Electronic Sum-Frequency Generation: A Unified View to Predict Interfacial pH of Biomembranes

For biomembranes, which are composed of neutral as well as charged lipids, the local pH at lipid/water interfaces is extremely important in their structural formation and functional activity. In our previous study of the charged lipid/water interfaces, we found that the local pH at the interface is governed by the positive or negative sign of the charge of the lipid: i.e., the local pH is dictated by the repulsive or attractive electrostatic interaction between the charged lipid headgroup and the proton. Because of the lack of net charge in the headgroup of the neutral lipid, the factor determining the local pH at neutral lipid/water interfaces is less straightforward, and therefore it is more challenging to predict the local pH. Here we apply heterodyne-detected electronic sum frequency generation (HD-ESFG) spectroscopy to nonionic and zwitterionic lipids to investigate the local pH at the neutral lipid/water interfaces. The obtained results indicate that the local pH at the nonionic lipid/water interface is higher than in bulk water by 0.8 whereas the local pH at the zwitterionic lipid/water interface is lower by 0.6, although the latter is subject to significant uncertainty. The present HD-ESFG study on neutral lipids, combined with the previous study on charged lipids, presents a unified view to consider the local pH at biomembranes based on the balance between the electrostatic interaction and the hydrophobicity provided by the lipid.

Accurate phase detection in time-domain heterodyne SFG spectroscopy

Heterodyne detection is a ubiquitous tool in spectroscopy for the simultaneous detection of intensity and phase of light. However, the need for phase stability hinders the application of heterodyne detection to electronic spectroscopy. We present an interferometric design for a phase-sensitive electronic sum frequency generation (e-SFG) spectrometer in the time domain with lock-in detection. Our method of continuous phase modulation of one arm of the interferometer affords direct measurement of the phase between SFG and local oscillator fields. Errors in the path length difference caused by drifts in the optics are corrected, offering unprecedented stability. This spectrometer has the added advantage of collinear fundamental beams. The capabilities of the spectrometer are demonstrated with proof-of-principle experiments with GaAs e-SFG spectra, where we see significantly improved signal to noise ratio, spectral accuracy, and lineshapes.

Revealing the Molecular Physics of Lattice Self-Assembly by Vibrational Hyperspectral Imaging

Lattice self-assemblies (LSAs), which mimic protein assemblies, were studied using a new nonlinear vibrational imaging technique called vibrational sum-frequency generation (VSFG) microscopy. This technique successfully mapped out the mesoscopic morphology, microscopic geometry, symmetry, and ultrafast dynamics of an LSA formed by β-cyclodextrin (β-CD) and sodium dodecyl sulfate (SDS). The spatial imaging also revealed correlations between these different physical properties. Such knowledge shed light on the functions and mechanical properties of LSAs. In this Feature Article, we briefly introduce the fundamental principles of the VSFG microscope and then discuss the in-depth molecular physics of the LSAs revealed by this imaging technique. The application of the VSFG microscope to the artificial LSAs also paved the way for an alternative approach to studying the structure-dynamic-function relationships of protein assemblies, which were essential for life and difficult to study because of their various and complicated interactions. We expect that the hyperspectral VSFG microscope could be broadly applied to many noncentrosymmetric soft materials.

Wedge-Based Design for Phase Stable and Phase Accurate Heterodyne-Detected Sum-Frequency Generation Spectroscopy

Phase sensitive and heterodyne-detected (HD) sum-frequency generation (SFG) spectroscopy offers the ability to separate the nonlinear susceptibility into its real and imaginary components. This provides information about the absolute orientation of molecules at interfaces while also producing an absorptive spectrum that is linear in spectral composition and can easily be decomposed into different spectral components. However, simultaneously obtaining phase accuracy and phase stability remains a challenge in SFG. Here we present a new experimental design for HD-SFG spectroscopy that incorporates a wedge pair to accurately control the timing between the local oscillator and the sample signal. This experimental approach provides high phase accuracy and long-time phase stability in a compact and flexible configuration.

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.

Deprotonation of Phenol linked to a silicon dioxide surface using Adaptive Feedback Laser Control with a Heterodyne Detected Sum Frequency Generation Signal

The development of laser-controlled surface reactions has been limited by the lack of decisive methods for detecting evolving changes in the surface chemistry. In this work, we demonstrate successful laser...

Time-resolved electronic sum-frequency generation spectroscopy with fluorescence suppression using optical Kerr gating

Excited state dynamics of molecules at interfaces can be studied using second-order non-linear spectroscopic methods such as time-resolved electronic sum-frequency generation (SFG). However, as such measurements inherently generate very small signals, they are often overwhelmed by signals originating from fluorescence. Here, this limitation is overcome by optical Kerr gating of the SFG signal to discriminate against fluorescence. The new approach is demonstrated on the excited state dynamics of malachite green at the water/air interface, in the presence of a highly fluorescent coumarin dye, and on the photo-oxidation of the phenolate anion at the water/air interface. The generality of the use of optical Kerr gating to SFG measurements is discussed.

Time-Variable Chiroptical Vibrational Sum-Frequency Generation Spectroscopy of Chiral Chemical Solution

Vibrational sum-frequency generation (VSFG) spectroscopy, a surface-specific technique, was shown to be useful even for characterizing the vibrational optical activity of chiral molecules in isotropic bulk liquids. However, accurately determining the spectroscopic parameters is still challenging because of the spectral congestion of chiroptical VSFG peaks with different amplitudes and phases. Here, we show that a time-variable infrared-visible chiroptical three-wave-mixing technique can be used to determine the spectroscopic parameters of second-order vibrational response signals from chiral chemical liquids. For varying the delay time between infrared and temporally asymmetric visible laser pulses, we measure the chiral VSFG, achiral VSFG, and their interference spectra of bulk R-(+)-limonene liquid and perform a global fitting analysis for those time-variable spectra to determine their spectroscopic parameters accurately. We anticipate that this time-variable VSFG approach will be useful for developing nearly background-free chiroptical characterization techniques with enhanced spectral resolution.

Molecular Structure and Surface Accumulation Dynamics of Hyaluronan at the Water-Air Interface

Hyaluronan is a biopolymer that is essential for many biological processes in the human body, like the regulation of tissue lubrication and inflammatory responses. Here, we study the behavior of hyaluronan at aqueous surfaces using heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSFG). Low-molecular-weight hyaluronan (∼150 kDa) gradually covers the water–air interface within hours, leading to a negatively charged surface and a reorientation of interfacial water molecules. The rate of surface accumulation strongly increases when the bulk concentration of low-molecular-weight hyaluronan is increased. In contrast, high-molecular-weight hyaluronan (>1 MDa) cannot be detected at the surface, even hours after the addition of the polymer to the aqueous solution. The strong dependence on the polymer molecular weight can be explained by entanglements of the hyaluronan polymers. We also find that for low-molecular-weight hyaluronan the migration kinetics of hyaluronan in aqueous media shows an anomalous dependence on the pH of the solution, which can be explained from the interplay of hydrogen bonding and electrostatic interactions of hyaluronan polymers.

Direct Evidence for a Surface and Bulk Specific Response in the Sum-Frequency Generation Spectrum of the Water Bend Vibration

We study the bending mode of pure water and charged aqueous surfaces using heterodyne-detected vibrational sum-frequency generation spectroscopy. We observe a low (1626  cm^{-1}) and a high (1656  cm^{-1}) frequency component that can be unambiguously assigned to an interfacial dipole and a bulk quadrupolar response, respectively. We thus demonstrate that probing the bending mode provides structural and quantitative information on both the surface and the bulk.

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.

Distinguishing different excitation pathways in two-dimensional terahertz-infrared-visible spectroscopy

In condensed molecular matter, low-frequency modes (LFMs) associated with specific molecular motions are excited at room temperature and determine essential physical and chemical properties of materials. LFMs, with typical mode energies of up to ∼500 cm^-1 (62 meV), contribute significantly to thermodynamic parameters and functions (e.g., heat capacity and entropy) and constitute the basis for room temperature molecular dynamics (e.g., conformational fluctuations and change). LFMs are often analyzed indirectly by the measurement of their effect on specific high-frequency modes (HFMs); the LFM-HFM coupling is reflected in the lineshape, as well as in the spectral and angular diffusion of the HFM. Two-dimensional terahertz-infrared-visible (2D TIRV) spectroscopy allows measuring the LFM-HFM coupling directly and can thereby provide new insights into the strength and nature of the coupling and the character of LFMs. However, the interference between the different signals generated by different excitation pathways can complicate 2D TIRV spectra, preventing a straightforward analysis. Here, we develop an experimental method to distinguish different excitation pathways in 2D TIRV spectroscopy and plot them separately in different quadrants of a 2D spectrum. We validate this method by measuring the spectra of CaF₂ and nitrogen gas. For CaF₂, only sum-frequency mixing between infrared and terahertz fields generates the signal. In contrast, for N₂, only difference-frequency mixing is observed. We then use this method to separate sum- and difference-frequency pathways in the 2D TIRV spectrum of liquid water, verifying the previous interpretation of the lineshape of the 2D TIRV spectrum of water.

The pH-dependent orientation of N3 dye on a gold substrate is revealed using heterodyne-detected vibrational sum frequency generation spectroscopy

We report on systematic changes to the adsorption geometry of the dye N3 {[cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato ruthenium(II)]} on a gold substrate as the pH of the deposition environment is altered. The protonation states of the four -COOH groups of the N3 dye change according to the modified pH conditions, thus affecting the number of -COOH and -NCS functional groups that participate in the adsorption to gold. Here, we use heterodyne detected vibrational sum frequency generation (HD-VSFG) spectroscopy to obtain surface specific vibrational information on both -COOH and -NCS groups as a function of pH of the deposition conditions. Polarization-dependent HD-VSFG yields sets of complex χ⁽²⁾ spectra, enabling us to perform a simultaneous fitting procedure to the polarization-dependent real and imaginary components and thus extract detailed structural information of the N3/gold interface. Our results show that N3 preferentially adsorbs to gold either with two -COOH groups and one -NCS group in more acidic conditions or with one -COOH group and two -NCS groups in more basic conditions.

Sensitivity of sum frequency generation experimental conditions to thin film interference effects

Sum-frequency generation (SFG) spectroscopy has furthered our understanding of the chemical interfaces that guide key processes in biology, catalysis, environmental science, and energy conversion. However, interpreting SFG spectra of systems containing several internal interfaces, such as thin film electronics, electrochemical cells, and biofilms, is challenging as different interfaces within these structures can produce interfering SFG signals. One potential way to address this issue is to carefully select experimental conditions that amplify the SFG signal of an interface of interest over all others. In this report, we investigate a model two-interface system to assess our ability to isolate the SFG signal from each interface. For SFG experiments performed in a reflective geometry, we find that there are few experimental conditions under which the SFG signal originating from either interface can be amplified and isolated from the other. However, by performing several measurements under conditions that alter their interference, we find that we can reconstruct each signal even in cases where the SFG signal from one interface is more than an order of magnitude smaller than its counterpart. The number of spectra needed for this reconstruction varies depending on the signal-to-noise level of the SFG dataset and the degree to which different experiments in a dataset vary in their sensitivity to each interface. Taken together, our work provides general guidelines for designing experimental protocols that can isolate SFG signals stemming from a particular region of interest within complex samples.

Quadrupole Contribution of C═O Vibrational Band in Sum Frequency Generation Spectra of Organic Carbonates

Sum Frequency Generation (SFG) is usually governed by surface-selective signals of dipole origin, but it can also contain some bulk signals of quadrupole origin. In this work, we examined the dipole and quadrupole contributions in the C═O stretching band of organic carbonate liquids with collaboration of heterodyne SFG measurement and theoretical analysis. As a result, we found that these spectra are substantially affected by the quadrupole contribution of the bulk, which resolved the discrepancy between the experimental and computational SFG spectra.

Phase-Sensitive Second-Harmonic Generation of Electrochemical Interfaces

The interaction between molecular species and charged interfaces plays an indispensable role in a multitude of electrochemical devices. Yet, very little is understood about the nature of this interaction, in particular, the interfacial electric field. Second-order nonlinear spectroscopy such as second-harmonic generation (SHG) can provide chemical information on these interfacial interactions; however, the phase information has received limited attention in electrochemical SHG studies. Here, we demonstrate that the phase of the SHG is essential to the measurement of the electric field at the electrode-electrolyte interface. Our in situ SHG experiment provides strong evidence in support of the parabolic model with complex nonlinear susceptibilities. We conclude that if the absolute phase of the total SHG signal with both χ⁽²⁾ and χ⁽³⁾ contributions can be obtained, it would be possible to measure the potential of zero charge of any electrochemical material.

Structure of water and polymer at the buried polymer/water interface unveiled using heterodyne-detected vibrational sum frequency generation

Heterodyne-detected vibrational sum frequency generation reveals the molecular-level structure of the polymer/water interface that is different from what has been argued.

Hydrogen order at the surface of ice Ih revealed by vibrational spectroscopy

A combination of heterodyne-detected sum frequency generation spectroscopy and theoretical modeling elucidates that the surface of ice I_h at 100 K has hydrogen order with the OH group pointing upward to the air (“H-up” orientation).

Orientational ordering in heteroepitaxial water ice on metal surfaces

Sum frequency generation spectroscopy uncovers the orientational ordering in crystalline ice films of water grown on Pt(111) and Rh(111).

A General Approach To Combine the Advantages of Collinear and Noncollinear Spectrometer Designs in Phase-Resolved Second-Order Nonlinear Spectroscopy

Recent years have seen a huge progress in the development of phase-sensitive second-order laser spectroscopy which has proven to be a very powerful tool for the investigation of interfaces. In these techniques, the nonlinear interaction between two short laser pulses and the sample yields a signal pulse which subsequently interferes with a third pulse, the so-called local oscillator. To obtain accurate phase information, the relative phases between the signal and local oscillator pulses must be stabilized and their timings precisely controlled. Despite much progress made, fulfilling both requirements remains a formidable experimental challenge. The two common approaches employ different beam geometries which each yields its particular advantages and deficiencies. While noncollinear spectrometers allow for a relatively simple timing control they typically yield poor phase stability and require a challenging alignment. Collinear approaches in contrast come with a simplified alignment and improved phase stability but typically suffer from a highly limited timing control. In this contribution we present a general experimental solution which allows for combining the advantages of both approaches while being compatible with most of the common spectrometer types. On the basis of a collinear geometry, we exploit different selected polarization states of the light pulses in well-defined places in the spectrometer to achieve a precise timing control. The combination of this technique with a balanced detection scheme allows for the acquisition of highly accurate phase-resolved nonlinear spectra without any loss in experimental flexibility. In fact, we show that the implementation of this technique allows us to employ advanced pulse timing schemes inside the spectrometer, which can be used to suppress nonlinear background signals and extend the capabilities of our spectrometer to measure phase-resolved sum frequency spectra of interfaces in a liquid cell.

Chirality discrimination at the carvone air/liquid interfaces detected by heterodyne-detected sum frequency generation

The chiral signal of the carvone air/liquid interface is probed by heterodyne-detected sum frequency generation (HD-SFG) without the electronic resonance. The chiral SFG spectra exhibit two distinguishable spectral signatures. Four chiral vibrational peaks of the R- and S-carvone molecules are with opposite signs, which can directly determine the surface molecular chirality. Two achiral vibrational peaks are also observed with the same sign. The different spectral signatures can provide a detailed chirality characterization at the molecular interface.

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.

Vibrational sum-frequency generation spectroscopy of electrode surfaces: studying the mechanisms of sustainable fuel generation and utilisation

The electrocatalytic oxidation of water coupled to the reduction of carbon dioxide, to make carbon based products, or the reduction of protons to provide hydrogen, offers a sustainable route to generating useful fuels.

Two dimensional crowding effects on protein folding at interfaces observed by chiral vibrational sum frequency generation spectroscopy

The crowding effect is prevalent in cellular environments due to high concentrations of biomacromolecules. It can alter the structures and dynamics of proteins and thus impact protein functions. The crowding effect is important not only in 3-dimensional cytoplasm but also for a 2-dimensional (2D) cell surface due to the presence of membrane proteins and glycosylation of membrane proteins and phospholipids. These proteins and phospholipids - with limited translational degrees of freedom along the surface normal - are confined in 2D space. Although the crowding effect at interfaces has been studied by adding crowding agents to bulk solution, the 2D crowding effect remains largely unexplored. This is mostly due to challenges in controlling 2D crowding and synergistic use of physical methods for in situ protein characterization. To address these challenges, we applied chiral vibrational sum frequency generation (SFG) spectroscopy to probe the sp1 zinc finger (ZnF), a 31-amino acid protein, folding into a β-hairpin/α-helix (ββα) motif upon binding to Zn2+. We anchored ZnF at the air/water interface via covalent linkage of ZnF to palmitic acid and controlled 2D crowding by introducing neutral lipid as a spacer. We obtained chiral amide I SFG spectra upon addition of Zn2+ and/or spacer lipid. The chiral SFG spectra show that interfacial crowding in the absence of spacer lipid hinders ZnF from folding into the ββα structure even in the presence of Zn2+. The results establish a paradigm for future quantitative, systematic studies of interfacial crowding effects.

Reduced Near-Resonant Vibrational Coupling at the Surfaces of Liquid Water and Ice

We study the resonant interaction of the OH stretch vibrations of water molecules at the surfaces of liquid water and ice using heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy. By studying different isotopic mixtures of H₂O and D₂O, we vary the strength of the interaction, and we monitor the resulting effect on the HD-SFG spectrum of the OH stretch vibrations. We observe that the near-resonant coupling effects are weaker at the surface than in the bulk, for both water and ice, indicating that for both phases of water the OH vibrations are less strongly delocalized at the surface than in the bulk.