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Quantifying Double-Layer Potentials at Liquid–Gas Interfaces from Vibrational Sum-Frequency Generation
摘要： Vibrational sum-frequency generation (SFG) spectroscopy is demonstrated as a fast method to quantify variations of the electric double-layer potential ?0 at liquid?gas interfaces. For this, mixed solutions of nonionic tetraethyleneglycol-monodecylether (C10E4) and cationic hexadecyltrimethylammonium bromide (C16TAB) surfactants were investigated using SFG spectroscopy and a thin-film pressure balance (TFPB). Derjaguin?Landau?Verwey?Overbeek analysis of disjoining pressure isotherms obtained with the TFPB technique provides complementary information on ?0, which we apply to validate the results from SFG spectroscopy. By using a single ?0 value, we can disentangle χ(2) and χ(3) contributions to the O?H stretching modes of interfacial water molecules in the SFG spectra. Having established the latter, we show that unknown double-layer potentials at the liquid?gas interface from solutions with different C16TAB/C10E4 mixing ratios can be obtained from an analysis of SFG spectra and are in excellent agreement with the complementary results from the TFPB technique.
关键词： disjoining pressure,Vibrational sum-frequency generation,electric double-layer potential,thin-film pressure balance,liquid?gas interfaces,surfactants
更新于2025-09-23 15:22:29
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Bulk-or-interface assignment of heterodyne-detected chiral vibrational sum frequency generation signal by its polarization dependence
摘要： Polarization dependence of heterodyne-detected chiral vibrational sum frequency generation (VSFG) was examined for thin films of polylactic acids and neat limonene liquid far from electronic resonance. The enantiomers of polylactic acid films on silica substrates were successfully distinguished, and their chiral VSFG signals were ascribed not to bulk but to the interfaces by comparing chiral signals observed in reflection in the S-polarized VSFG, P-polarized visible, and P-polarized infrared and P-polarized VSFG, S-polarized visible, and P-polarized infrared polarization combinations with theoretical model calculations. In the same way, the chiral VSFG signal of neat limonene was assigned to bulk, which is consistent with the previous assignment. The method employed for assigning the source of chiral signals to the bulk or the interface may be useful for organic films on substrates with low refractive indices and thick samples.
关键词： limonene,chiral vibrational sum frequency generation,polarization dependence,bulk-or-interface assignment,heterodyne detection,polylactic acid
更新于2025-09-23 15:22:29
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Hidden Isolated OH at the Charged Hydrophobic Interface Revealed by Two-Dimensional Heterodyne-Detected VSFG Spectroscopy
摘要： Water around hydrophobic groups mediates hydrophobic interactions that play key roles in many chemical and biological processes. Thus, the molecular-level elucidation of the properties of water in the vicinity of hydrophobic groups is important. We report on the structure and dynamics of water at two oppositely charged hydrophobic ion/water interfaces, the tetraphenylborate-ion (TPB?)/water and tetraphenylarsonium-ion (TPA+)/water interfaces, which are clarified by two-dimensional heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) spectroscopy. The obtained 2D HD-VSFG spectra of the anionic TPB? interface reveal the existence of distinct π-hydrogen bonded OH groups in addition to the usual hydrogen-bonded OH groups, which are hidden in the steady-state spectrum. In contrast, 2D HD-VSFG spectra of the cationic TPA+ interface only show the presence of usual hydrogen-bonded OH groups. The present study demonstrates that the sign of the interfacial charge governs the structure and dynamics of water molecules that face the hydrophobic region.
关键词： hydrophobic interfaces,sum-frequency generation,ultrafast spectroscopy,interfacial water,vibrational dynamics
更新于2025-09-23 15:21:01
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Origin of the Overpotential for the Oxygen Evolution Reaction on a Well-defined Graphene Electrode Probed by in situ Sum Frequency Generation Vibrational Spectroscopy
摘要： To develop an efficient material for the cathode of the lithium-oxygen (Li-O2) secondary battery, the oxygen reduction and evolution reactions (ORR and OER) on a well-defined graphene monolayer have been investigated in a typical organic solvent, dimethyl sulfoxide (DMSO). The adsorption and desorption behaviors of the solvents on the graphene electrode surface were evaluated by an intrinsically surface-selective vibrational spectroscopy of sum frequency generation (SFG) during the ORR and OER. After the initial ORR depositing lithium peroxide (Li2O2) on the graphene electrode surface in a LiClO4/DMSO solution, the SFG spectroscopy revealed that the subsequent OER oxidizing the Li2O2 preferentially proceeds at the interface between the Li2O2 and graphene rather than that between the Li2O2 and bulk solution. Therefore, the OER tends to reduce the electric conductivity between the Li2O2 and graphene by decreasing their contact area before a large part of the deposited Li2O2 was oxidized, which elucidates the origin of the high overpotential for the OER.
关键词： oxygen reduction reaction,lithium-oxygen battery,oxygen evolution reaction,sum frequency generation vibrational spectroscopy,graphene electrode
更新于2025-09-23 15:21:01
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Broadband models and their consequences on line shape analysis in vibrational sum-frequency spectroscopy
摘要： Vibrational sum-frequency generation (SFG) spectroscopy can provide valuable qualitative and quantitative information about molecular species at surface and buried interfaces. For example, the resonance frequency of a particular chemical function group is revealing of the surface environment, especially when compared to what is observed in bulk IR absorption or Raman scattering spectra. Furthermore, the amplitude of the mode can be related to the molecular orientation, providing a detailed quantitative account of the surface structure. Each of these attributes, however, requires fitting the spectra to some vibrationally resonant line shape. This is particularly challenging when the modes of interest co-exist with broad resonance features, such as water O–H stretching. In this perspective, we examine the merits and consequences of different approaches to fitting homodyne SFG data. We illustrate that, while any model can provide a useful description of the data, no model can accurately and consistently provide even the relative phase deeply encoded in homodyne data without the use of additional information.
关键词： Vibrational sum-frequency generation,homodyne data,surface structure,molecular orientation,line shape analysis,SFG spectroscopy
更新于2025-09-23 15:21:01
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Hidden isolated OH at the charged hydrophobic interface revealed by two-dimensional heterodyne-detected VSFG spectroscopy
摘要： Water around the hydrophobic groups mediates hydrophobic interaction that plays key roles in many chemical and biological processes. Thus, molecular-level elucidation of the properties of the water in the vicinity of the hydrophobic group is important. We report on the structure and dynamics of water at two oppositely charged hydrophobic interfaces, i.e., the tetraphenylborate ion (TPB-)/water and tetraphenylarsonium ion (TPA+)/water interfaces, which are clarified by two-dimensional heterodyne-detected vibrational sum frequency generation (2D HD-VSFG) spectroscopy. The obtained 2D HD-VSFG spectra of anionic TPB- interface reveal the existence of distinct π-hydrogen bonded OH groups in addition to the usual hydrogen bonded OH groups, which is hidden in the steady-state spectrum. In contrast, 2D HD-VSFG spectra of cationic TPA+ interface only show the presence of usual hydrogen bonded OH groups. The present study demonstrates that the sign of the interfacial charge governs the structure and dynamics of water molecules that face the hydrophobic region.
关键词： sum-frequency generation,ultrafast spectroscopy,hydrophobic interfaces,interfacial water,vibrational dynamics
更新于2025-09-23 15:19:57
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Determination of the electric field and its Hilbert transform in femtosecond electro-optic sampling
摘要： We demonstrate time-domain sampling of mid-infrared electric ?eld transients and their conjugate counterparts exploiting the dynamical Pockels effect. To this end, the complete polarization change of few-femtosecond probe pulses is studied. An intuitive picture based on a phasor representation is established before gaining quantitative understanding in experiment and theory. In the standard version of electro-optic sampling, the electric ?eld is determined by analyzing the change of ellipticity of the probe polarization. Beyond this, we ?nd that a temporal gradient of the input electric ?eld manifests itself in a rotation of the polarization ellipsoid of the probe. The relative contribution of sum- and difference-frequency mixing processes and their spectral distribution over the near-infrared probe bandwidth are identi?ed as key aspects. If one of these processes dominates, detecting ellipticity changes and polarization rotation as a function of time delay results in two wave forms which are Hilbert transforms of each other. Such conditions may be achieved by angle phase matching in birefringent materials or spectral ?ltering of the probe after the nonlinear interaction. In this case, a static phase introduced by birefringence or re?ection at metallic mirrors results in a speci?c phase shift of both time traces with respect to the input electric ?eld. Contributions from sum- and difference-frequency generation are found to be equivalent when using electro-optic sensors with isotropic refractive index. Polarization rotations in the low- and high-frequency parts of the probe then tend to cancel out. In this limit, spurious additional phase shifts do not change the phase of the detected transients. This fact leads to a robust recovery of the carrier-envelope phase of the input wave form. Clarifying the role of imperfections of superachromatic phase retarders completes our survey on proper determination of the electric ?eld and its conjugate variable.
关键词： polarization rotation,sum-frequency generation,difference-frequency generation,electro-optic sampling,Pockels effect,mid-infrared,Hilbert transform
更新于2025-09-19 17:13:59
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Time-resolved measurements of two-color laser light emitted from GaAs/AlGaAs-coupled multilayer cavity
摘要： We measured the two-color laser oscillation from a GaAs/AlGaAs-coupled multilayer cavity at 18 °C–42 °C using current injection. We confirmed simultaneous lasing by detecting the sum frequency generation signal generated by the two-color laser light, and performed time-resolved measurement using a streak camera with a spectrometer. From the observed time transient of the spectra at various temperatures, it is clarified that the temperature change of the device, induced by current injection, modulates the effective cavity length. Therefore, the temperature control of the device is a key factor in stable two-color lasing and THz wave generation.
关键词： time-resolved measurements,sum frequency generation,THz wave generation,GaAs/AlGaAs-coupled multilayer cavity,two-color laser
更新于2025-09-19 17:13:59
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Hydrogen Bonds and Molecular Orientations of Supramolecular Structure between Barbituric Acid and Melamine Derivative at the Air/Water Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy
摘要： We studied the supramolecular structure between barbituric acid (pyrimidine-2,4,6(1H,3H,5H)-trione, BA) and an amphiphilic melamine derivative at the air/water interface by heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. HD-VSFG measurements in situ showed a positive broad band from 2300 to 2950 cm?1. By comparing the experimental results with ab initio molecular dynamics (AIMD) simulations, we assigned the broad band to the NH stretching modes of BA strongly hydrogen-bonded to the melamine derivative. In addition, we report in situ HD-VSFG spectra of the interfacial supramolecular structure in the CO stretching region. Two CO stretching bands were identified. On the basis of the signs of the C=O bands, we uniquely determined the orientation of BA. The strong hydrogen bonds and the molecular orientations are direct evidence for the supramolecular structure based on complementary hydrogen bonds at the air/water interface.
关键词： Supramolecular Structure,Molecular Orientations,Barbituric Acid,Melamine Derivative,Hydrogen Bonds,Air/Water Interface,Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy
更新于2025-09-19 17:13:59
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Retaining 3D Laser Pulse Shape at Sum Frequency Generation Processes
摘要： Modern photoinjectors require to use 3D profiled laser pulses for irradiation of cathode surface. The pulses make it possible to control a space-charge distribution of generated electron bunches and, in particular, to form electron bunches with cylindrical and 3D ellipsoidal shapes. Spatial light modulators (SLM) [1,2] and profiled volume Bragg gratings [3] can be used for control 3D intensity distribution of linearly chirped broadband infrared laser pulses. But, the widely used in photoinjectors Cs2Te photocathodes have the highest efficiency in UV spectral range. So, the important task is to transform the profiled 3D (x,y,t) infrared pulses to visible and UV regions with high energy conversion efficiency and preserving 3D intensity distribution. It can be done with help of second, fourth and third harmonic generation (SHG, FHG, THG) processes at low group velocity mismatch of the interacted pulses. The control of the group velocities can be done by a creation of amplitude tilt or angular chirp. Here we present results of numerical simulations of SHG, FHG and noncollinear THG processes implemented for laser pulses with central wavelength 744 nm, 6 nm spectral width (FWHM) and pulse duration 15 ps (FWHM). The initial fundamental pulse has ellipsoidal 3D shape with linear growth intensity in time.
关键词： energy conversion efficiency,3D laser pulse shape,sum frequency generation,harmonic generation,photoinjectors
更新于2025-09-16 10:30:52