Generation Vibrational Spectra Research Articles

Temporal and chirp effects of laser pulses on the spectral line shape in sum-frequency generation vibrational spectroscopy.

Assignment and interpretation of the sum-frequency generation vibrational spectra (SFG-VS) depend on the ability to measure and understand the factors affecting the SFG-VS spectral line shape accurately and reliably. In the past, the formulation of the polarization selection rules for SFG-VS and the development of the sub-wavenumber high-resolution broadband SFG-VS (HR-BB-SFG-VS) have provided solutions for many of these needs. However, despite these advantages, HR-BB-SFG-VS have not been widely adopted. The majority of SFG measurements so far still relies on the picosecond (ps) scanning SFG-VS or the conventional broadband SFG-VS (BB-SFG-VS) with the spectral resolution around (mostly above) 10cm-1, which also results in less ideal spectral line shape in the SFG spectra due to the temporal and chirp effects of the laser pulses used in experiment. In this study, the temporal and the chirp effects of laser pulses with different profiles in the SFG experiment on the measured SFG-VS spectral line shape are examined through spectral simulation. In addition, the experimental data of a classical model system, i.e., octadecyltrichlorosilane monolayer on glass, obtained from the ps scanning SFG-VS, the BB-SFG-VS, and the HR-BB-SFG-VS measurements are directly compared and examined. These results show that temporal and chirp effects are often significant in the conventional BB-SFG-VS, resulting in line shape distortions and peak position shifts besides spectral broadening. Such temporal and chirp effects are less significant in the ps scanning SFG-VS. For the HR-BB-SFG-VS, spectral broadening and temporal and chirp effects are insignificant, making HR-BB-SFG-VS the choice for accurate and reliable measurement and analysis of SFG-VS.

Comprehensive analysis on anomalous phenomenon of ethanol‐soluble poly(vinyl butyral) membrane for ethanol recovery via pervaporation

AbstractPoly(vinyl butyral) (PVB) is selected as a promising ethanol‐permselective membrane based on the solubility parameter theory; however, it exhibits anomalous water perm‐selectivity in practical pervaporation (PV) process. Comprehensive analysis based on experimental and theoretical methods were carried out to explore the inherent mechanism of the anomalous performance. Firstly, sum frequency generation vibrational spectra and contact angle were developed to quantify the surface reconstruction of membrane in air and ethanol, which indicated that hydrophilic hydroxyl tended to expose on membrane surface with ethanol thus improving the membrane affinity to water. Meanwhile, swelling behaviors proved more water would accumulate in the ethanol swollen membrane. Furthermore, theoretical analysis in terms of sorption and diffusion process, based on the UNIFAC‐FV model and Fujita free‐volume theory, confirmed the mechanism of anomalous phenomenon of PVB membrane. The comprehensive investigation was expected to provide insights into the basic separation mechanism of PV process.

Sum frequency generation vibrational spectra of perovskite nanocrystals at the single-nanocrystal and ensemble levels

Determination of molecular structures of organic-inorganic hybrid perovskite (OIHP) nanocrystals at the single-nanocrystal and ensemble levels is essential to understanding the mechanisms responsible for their size-dependent optoelectronic properties and the nanocrystal assembling process, but its detection is still a bit challenging. In this study, we demonstrate that femtosecond sum frequency generation (SFG) vibrational spectroscopy can provide a highly sensitive tool for probing the molecular structures of nanocrystals with a size comparable to the Bohr diameter (∼10 nm) at the single-nanocrystal level. The SFG signals are monitored using the spectral features of the phenyl group in (R-MBA)PbBr3 and (R-MBA)2PbI4 nanocrystals (MBA: methyl-benzyl-ammonium). It is found that the SFG spectra exhibit a strong resonant peak at 3067±3 cm−1 (ν2 mode) and a weak shoulder peak at 3045±4 cm−1 (ν7a mode) at the ensemble level, whereas a peak of the ν2 mode and a peak at 3025±3 cm−1 (ν20b mode) at the single-nanocrystal level. The nanocrystals at the single-nanocrystal level tend to lie down on the surface, but stand up as the ensemble number and the averaged sizes increase. This finding may provide valuable information on the structural origins for size-dependent photo-physical properties and photoluminescence blinking dynamics in nanocrystals.

Vibrational Sum-Frequency Generation Spectroscopy in the Energy Representation from Dual-Level Molecular Dynamics Simulations.

We report a cost-effective molecular dynamics approach to calculate sum-frequency generation (SFG) vibrational spectra of molecular species at liquid interfaces in the energy representation formalism that brings together the instantaneous normal mode (INM) analysis at free-energy minima (FEM) and the dual-level free-energy perturbation (FEP) methods. This combined FEP-INM-FEM approach allows analyzing SFG spectra in terms of normal mode contributions at very-high ab initio levels, in contrast to standard time-correlation function (TCF)-based methods, from which it can be considered complementary. It is applied here to the study of the CH3-stretching band of methanol at the air-water interface, which has been thoroughly studied in the literature. The SFG band of acetonitrile in the same conditions has also been calculated with the aim of testing the capability of the method to reproduce small chemical shifts. The suitability of the proposed model is demonstrated by comparing the results with TCF data from QM/MM simulations and with experiments. Analysis of these results provides new insights into the strength and orientational dependence of the SFG signal of symmetric and asymmetric stretching vibrations of CH3 groups, which can be of paramount importance to analyze the spectra of more complex systems.

Surface chemistry of the frog sticky-tongue mechanism.

Frogs capture their prey with a highly specialized tongue. Recent studies indicate this tongue is covered with fibril-forming mucus that acts as a pressure sensitive adhesive. However, no analysis of the interfacial chemistry of frog tongue mucus has been performed. The goal of this study is to examine the chemical structure of the surface of mucus after a tongue strike. Previous studies of mucus from other animals suggest that mucus from a frog's tongue consists of mucins-serine-, threonine-, and proline-rich glycoproteins. Therefore, the authors expect to observe chemical bonds associated with glycoproteins, as well as fibrils formed at the mucus-tongue interface. To test this hypothesis, they collected both near-edge x-ray absorption fine structure (NEXAFS) microscopy images and sum frequency generation (SFG) vibrational spectra from layers of mucus left after frog tongue strikes on cleaned glass slides. NEXAFS imaging demonstrates a uniform distribution of amide, hydroxyl, and carbon-carbon bonds across the mucus surface. Difference spectra of individual N1s and C1s K-edge spectra pulled from these images indicate a structure consistent with fibril formation as well as disorder of oligosaccharide groups near the mucus surface. C-H region SFG spectra reveal surface active modes which likely stem from serine and threonine within the mucin protein. Combined, this work suggests that glycoproteins are well-ordered at the mucus-tongue interface.

High-resolution broadband sum frequency generation vibrational spectroscopy using intrapulse interference.

We theoretically propose a new approach to obtain high-spectral-resolution sum frequency generation (SFG) vibrational spectra using intrapulse interference. By introducing a π-step phase modulation to the broadband 800 nm pulse, the broadband 800 nm laser pulse splits into two distinguishable pulses in the time domain with a fixed time delay. The resolution of the intrapulse interference SFG can be better than 1 cm-1 and is limited only by the spectral resolution of the spectrometer. This approach can accurately retrieve the amplitude and the relative phase of vibrational peaks. Additionally, the sensitivity of SFG is enhanced by adopting femtosecond IR and femtosecond visible pulses.

Homogeneous and inhomogeneous broadenings and the Voigt line shapes in the phase-resolved and intensity sum-frequency generation vibrational spectroscopy.

In this report, we show that the ability to measure the sub-1 cm(-1) resolution phase-resolved and intensity high-resolution broadband sum frequency generation vibrational spectra of the -CN stretch vibration of the Langmuir-Blodgett (LB) monolayer of the 4-n-octyl-4'-cyanobiphenyl (8CB) on the z-cut α-quartz surface allows the direct comparison and understanding of the homogeneous and inhomogeneous broadenings in the imaginary and intensity SFG vibrational spectral line shapes in detail. The difference of the full width at half maximum (FWHM) of the imaginary and intensity sum-frequency generation vibrational spectroscopy spectra of the same vibrational mode is the signature of the Voigt line shape and it measures the relative contribution to the overall line shape from the homogeneous and inhomogeneous broadenings in SFG vibrational spectra. From the phase-resolved and intensity spectra, we found that the FWHM of the 2238.00 ± 0.02 cm(-1) peak in the phase-resolved imaginary and intensity spectra is 19.2 ± 0.2 cm(-1) and 21.6 ± 0.4 cm(-1), respectively, for the -CN group of the 8CB LB monolayer on the z-cut α-quartz crystal surface. The FWHM width difference of 2.4 cm(-1) agrees quantitatively with a Voigt line shape with a homogeneous broadening half width of Γ = 5.29 ± 0.08 cm(-1) and an inhomogeneous standard derivation width Δω = 5.42 ± 0.07 cm(-1). These results shed new lights on the understanding and interpretation of the line shapes of both the phase-resolved and the intensity SFG vibrational spectra, as well as other incoherent and coherent spectroscopic techniques in general.

First-Principles Framework to Compute Sum-Frequency Generation Vibrational Spectra of Semiconductors and Insulators.

We present a first-principles framework to compute sum-frequency generation (SFG) vibrational spectra of semiconductors and insulators. The method is based on density functional theory and the use of maximally localized Wannier functions to compute the response to electric fields, and it includes the effect of electric field gradients at surfaces. In addition, it includes quadrupole contributions to SFG spectra, thus enabling the verification of the dipole approximation, whose validity determines the surface specificity of SFG spectroscopy. We compute the SFG spectra of ice I_{h} basal surfaces and identify which spectra components are affected by bulk contributions. Our results are in good agreement with experiments at low temperature.

Direct characterization of polymer encapsulated CdSe/CdS/ZnS quantum dots

Surface engineering advances of semiconductor quantum dots (QDs) have enabled their application to molecular labeling, disease diagnostics and tumor imaging. For biological applications, hydrophobic core/shell QDs are transferred into aqueous solutions through the incorporation of water-solubility imparting moieties, typically achieved via direct exchange of the native surface passivating ligands or indirectly through the adsorption of polymers. Although polymeric encapsulation has gained wide acceptance, there are few reports addressing the characterization of the adsorbed polymers and existing theoretical analyses are typically based on simple geometric models. In this work, we experimentally characterize and quantify water-soluble QDs prepared by adsorption of amphiphilic poly(maleic anhydride-alt-1-tetradecene) (PMAT, MW~9000) onto commercially available CdSe/CdS/ZnS (CdSe/CdS/ZnS-PMAT). Using X-ray photoelectron spectroscopy (XPS) we determined that ~15 PMAT molecules are adsorbed onto each QD and sum frequency generation (SFG) vibrational spectra were utilized to investigate the mechanism of interaction between PMAT molecules and the QD surface. Importantly, when employed together, these techniques constitute a platform with which to investigate any polymer–nanoparticle complex in general.

Influence of the linkage type between the polymer backbone and side groups on the surface segregation of methyl groups during film formation.

Although poly(vinyl acetate) (PVAc) differs from poly(methyl acrylate) (PMA) only in the reversed position of the ester group, a large difference in the concentration dependence of the casting solution on the corresponding surface structure of the cast films of PVAc, PMA and poly(methyl methacrylate) (PMMA) was observed. The hydrophobicity of both PMA and PMMA films increased with increasing concentration of the corresponding polymer solution, whereas cast PVAc films showed the reverse trend. The surface structure of the cast films prepared with different concentrations of the casting solution, characterized by sum frequency generation (SFG) vibrational spectra, showed that the order of the methylene groups increased while that of the acetyl methyl group decreased on the surface of cast PVAc film with increasing concentration of casting solution. However, the order of the ester methyl group increased and that of methylene groups did not change for cast PMA films with increasing concentration of casting solution. The cast PMMA film showed a reverse trend compared with the corresponding PMA film. It is apparent that well-ordered ester or acetyl methyl groups on the surface, which are oriented away from the polymer film, rather than methylene groups, play an important role in determining surface hydrophobicity, as the latter shield the OC[double bond, length as m-dash]O groups of PVAc, PMA and PMMA film surfaces from being exposed, resulting in low surface free energy. The reason for this difference is attributed to the relatively low energy for ester methyl group reorientation, an ester group structure nearer to the trans state and more regular local configuration of segments in concentrated solutions of PMA and PMMA compared to that of PVAc.

Effect of mechanical disruption on the effectiveness of three reactors used for dilute acid pretreatment of corn stover Part 1: chemical and physical substrate analysis

BackgroundThere is considerable interest in the conversion of lignocellulosic biomass to liquid fuels to provide substitutes for fossil fuels. Pretreatments, conducted to reduce biomass recalcitrance, usually remove at least some of the hemicellulose and/or lignin in cell walls. The hypothesis that led to this research was that reactor type could have a profound effect on the properties of pretreated materials and impact subsequent cellulose hydrolysis.ResultsCorn stover was dilute-acid pretreated using commercially relevant reactor types (ZipperClave® (ZC), Steam Gun (SG) and Horizontal Screw (HS)) under the same nominal conditions. Samples produced in the SG and HS achieved much higher cellulose digestibilities (88% and 95%, respectively), compared to the ZC sample (68%). Characterization, by chemical, physical, spectroscopic and electron microscopy methods, was used to gain an understanding of the effects causing the digestibility differences. Chemical differences were small; however, particle size differences appeared significant. Sum-frequency generation vibrational spectra indicated larger inter-fibrillar spacing or randomization of cellulose microfibrils in the HS sample. Simons’ staining indicated increased cellulose accessibility for the SG and HS samples. Electron microscopy showed that the SG and HS samples were more porous and fibrillated because of mechanical grinding and explosive depressurization occurring with these two reactors. These structural changes most likely permitted increased cellulose accessibility to enzymes, enhancing saccharification.ConclusionsDilute-acid pretreatment of corn stover using three different reactors under the same nominal conditions gave samples with very different digestibilities, although chemical differences in the pretreated substrates were small. The results of the physical and chemical analyses of the samples indicate that the explosive depressurization and mechanical grinding with these reactors increased enzyme accessibility. Pretreatment reactors using physical force to disrupt cell walls increase the effectiveness of the pretreatment process.

Effect of mechanical disruption on the effectiveness of three reactors used for dilute acid pretreatment of corn stover Part 2: morphological and structural substrate analysis

BackgroundLignocellulosic biomass is a renewable, naturally mass-produced form of stored solar energy. Thermochemical pretreatment processes have been developed to address the challenge of biomass recalcitrance, however the optimization, cost reduction, and scalability of these processes remain as obstacles to the adoption of biofuel production processes at the industrial scale. In this study, we demonstrate that the type of reactor in which pretreatment is carried out can profoundly alter the micro- and nanostructure of the pretreated materials and dramatically affect the subsequent efficiency, and thus cost, of enzymatic conversion of cellulose.ResultsMulti-scale microscopy and quantitative image analysis was used to investigate the impact of different biomass pretreatment reactor configurations on plant cell wall structure. We identify correlations between enzymatic digestibility and geometric descriptors derived from the image data. Corn stover feedstock was pretreated under the same nominal conditions for dilute acid pretreatment (2.0 wt% H2SO4, 160°C, 5 min) using three representative types of reactors: ZipperClave® (ZC), steam gun (SG), and horizontal screw (HS) reactors. After 96 h of enzymatic digestion, biomass treated in the SG and HS reactors achieved much higher cellulose conversions, 88% and 95%, respectively, compared to the conversion obtained using the ZC reactor (68%). Imaging at the micro- and nanoscales revealed that the superior performance of the SG and HS reactors could be explained by reduced particle size, cellular dislocation, increased surface roughness, delamination, and nanofibrillation generated within the biomass particles during pretreatment.ConclusionsIncreased cellular dislocation, surface roughness, delamination, and nanofibrillation revealed by direct observation of the micro- and nanoscale change in accessibility explains the superior performance of reactors that augment pretreatment with physical energy.

Effect of visible pulse shaping on the accuracy of relative intensity measurements in BBSFG vibrational spectroscopy

Quantitative analysis of BroadBand Sum Frequency Generation (BBSFG) vibrational spectra by deconvolution into Lorentzian components has been shown recently to suffer from complications that depend on the spectro-temporal properties and delay of the visible pulse which has to be shaped in order to achieve spectral resolution and possibly temporal discrimination. We present a comprehensive spectro-temporal analysis of BBSFG in order to evaluate quantitatively the consequences of delay dependent spectral changes on the accuracy of BBSFG spectra. We compare purely spectral deconvolution of single spectra and spectro-temporal analysis of multiple spectra obtained with a picosecond visible pulse produced by either a Fabry–Perot (FP) etalon or a 4f pulse shaper (PS). The case of OctaDecaneThiol Self Assembled Monolayer is used for comparison of theory and experiment. Accurate relative intensities can only be obtained from the analysis of multiple spectra generated by the FP-produced visible pulse. Although the spectra obtained with the PS are not delay dependent contrarily to those of the FP, the purely spectral deconvolution does not provide accurate values of the relative intensities. The need for a spectro-temporal analysis is explained by the fact that the first-order IR polarization is distorted both in the time domain by the ps visible pulse shape and in the frequency domain by interferences between the bands that are complicated by a frequency and delay dependent phase shift.

Probing the orientation of electrostatically immobilized cytochrome C by time of flight secondary ion mass spectrometry and sum frequency generation spectroscopy

By taking advantage of the electron pathway through the heme group in cytochrome c (CytoC) electrochemists have built sensors based upon CytoC immobilized onto metal electrodes. Previous studies have shown that the electron transfer rate through the protein is a function of the position of this heme group with respect to the electrode surface. In this study a detailed examination of CytoC orientation when electrostatically immobilized onto both amine (NH3+) and carboxyl (COO-) functionalized gold is presented. Protein coverage, on both surfaces, was monitored by the change in the atomic % N, as determined by x-ray photoelectron spectroscopy. Spectral features within the in situ sum frequency generation vibrational spectra, acquired for the protein interacting with positively and negatively charged surfaces, indicates that these electrostatic interactions do induce the protein into a well ordered film. Time of flight secondary ion mass spectrometry data demonstrated a clear separation between the two samples based on the intensity differences of secondary ions stemming from amino acids located asymmetrically within CytoC (cysteine: C2H6NS+; glutamic acid: C4H6NO+ and C4H8NO2+; leucine: C5H12N+). For a more quantitative examination of orientation, we developed a ratio comparing the sum of the intensities of secondary-ions stemming from the amino acid residues at either end of the protein. The 50 % increase in this ratio, observed between the protein covered NH3+ and COO- substrates, indicates opposite orientations of the CytoC on the two different surfaces.

Solvent quality affects chain conformational order at the polymer surface revealed by sum frequency generation vibrational spectroscopy

The surface sum frequency generation (SFG) vibrational spectroscopic spectra of PMA and PMMA films prepared from solvents with different qualities were found different. This suggests that the surface molecular structures of PMA and PMMA films were affected by the solvent quality. A simple phenomenological model was used to describe the chain conformational order at the polymer surface by correlating the SFG resonant signals of the side chromophoric groups to the introduced chain arc included angle. This model is believed to be applicable for polymers with short and rigid side chromophores active for the SFG spectroscopy.

Depth profile of the segmental dynamics at a poly(methyl methacrylate) film surface

We investigated the segmental mobility distribution at the poly(methyl methacrylate) film surface within the depth of 9.0 nm, by measuring the surface reorganization of fluorinated tracer-labeled PMMA using three different surface-sensitive techniques with various analytical depth capabilities, including contact angle measurements, sum frequency generation (SFG) vibrational spectra and angle-resolved X-ray photoelectron spectroscopy (XPS). The onset temperature of fluorinated chain end reorganization (Tonset) at various depths was identified and is shown to depend on the sampling depth of the techniques. The Tonset was verified to be related to the segmental relaxation of PMMA. Plotting the Tonset against the sensing depth of the corresponding technique, the distribution of surface Tonset within a 9.0 nm depth was obtained. An approximately 2.4 nm surface layer with depth-independent Tonset was found, below which the Tonset increased with increasing depth. The depth profile of Tonset effectively divides a film into three layers: a “surface layer” with Arrhenius dynamics on the uppermost film surface, a “gradient region” layer underneath the top layer transporting “liquid-like” dynamics into the internal film, and the remaining bulk layer. This observed depth profile of the surface dynamics may suggest that the polymer chains near the surface adopt an oblate conformation and have lowered entanglements.

Resolving Two Closely Overlapping −CN Vibrations and Structure in the Langmuir Monolayer of the Long-Chain Nonadecanenitrile by Polarization Sum Frequency Generation Vibrational Spectroscopy

Polarization sum frequency generation vibrational spectra (SFG-VS) reveals that there are two distinctively different but closely overlapping −CN vibrations at 2244.5 and 2251.1 cm–1, respectively, in the Langmuir monolayer of the long-chain nonadecanenitrile (CH3(CH2)17CN, or C18CN) at the air/water interface. The blue-shifted −CN group at the 2251.1 cm–1 peak is about 1.8 times broader than that of the 2244.5 cm–1. Both the spectral shift and spectral width are consistent with the picture that this blue-shifted peak corresponds to the solvated −CN group, while the 2244.5 cm–1 peak is the signature of the less solvated −CN group. Polarization dependence of these two peaks further suggests that the −CN group corresponding to the 2251.1 cm–1 peak is tilted with an average angle of 50° from interface normal, while that corresponding to the 2244.5 cm–1 peak is tilted with an angle around 67°. The relative population for the −CN groups corresponding to the 2251.1 cm–1 peak is about three times that of the 224...

Intramolecular vibrational coupling in water molecules revealed by compatible multiple nonlinear vibrational spectroscopic measurements

The complex structure of water and its interactions with solid surfaces require the development of multiple vibrational spectroscopic measurements to study the molecular structure of interfacial water and bulk water near the solid surface. In this study, a newly developed compatible multiple nonlinear vibrational spectroscopy system has been applied to investigate the molecular structure of water in the interfacial region of an ionic solid (CaF(2) substrate) and bulk isotopic D(2)O-HOD-H(2)O mixtures. Using this compatible system, the sum frequency generation (SFG) vibrational spectra and infrared-infrared-visible three-pump-field four-wave-mixing (IIV-TPF-FWM) spectra of the same water molecules can be collected at the same experimental geometry. It is found that SFG and IIV-TPF-FWM can be used to characterize the molecular structures of interfacial water and bulk water molecules at an interfacial distance below 42 nm, respectively. SFG and IIV-TPF-FWM results both suggest an intramolecular vibrational coupling dominates the spectra. The results achieved by this method are helpful to clarify the origination of the vibrational coupling of the interfacial water as well as the bulk water near the solid surface.

Molecular dynamics simulation of liquid methanol. II. Unified assignment of infrared, raman, and sum frequency generation vibrational spectra in methyl C–H stretching region

Vibrational spectra of methyl C-H stretching region are notoriously complicated, and thus a theoretical method of systematic assignment is strongly called for in condensed phase. Here we develop a unified analysis method of the vibrational spectra, such as infrared (IR), polarized and depolarized Raman, and ssp polarized sum frequency generation (SFG), by flexible and polarizable molecular dynamics simulation. The molecular model for methanol has been developed by charge response kernel model to allow for analyzing the methyl C-H stretching vibrations. The complicated spectral structure by the Fermi resonance has been unraveled by empirically shifting potential parameters, which provides clear information on the coupling mechanism. The analysis confirmed that for the IR, polarized Raman, and SFG spectra, two-band structure at about 2830 and 2950 cm(-1) results from the Fermi resonance splitting of the methyl C-H symmetric stretching and bending overtones. In the IR spectrum, the latter, higher-frequency band is overlapped with prominent asymmetric C-H stretching bands. In the depolarized Raman spectrum, the high frequency band at about 2980 cm(-1) is assigned to the asymmetric C-H stretching mode. In the SFG spectrum, the two bands of the splitted symmetric C-H stretching mode have negative amplitudes of imaginary nonlinear susceptibility χ(2), while the higher-frequency band is partly cancelled by positive imaginary components of asymmetric C-H stretching modes.

An approach to compatible multiple nonlinear vibrational spectroscopy measurements using a commercial sum frequency generation system

In this paper, we designed a compatible multiple nonlinear vibrational spectroscopy system that can be used for recording infrared-visible sum frequency generation vibrational spectra (SFG) and infrared-infrared-visible three-pump-field four-wave-mixing (IIV-TPF-FWM) spectra using a commercial EKSPLA SFG system. This is the first time IIV-TPF-FWM signals were obtained using picosecond laser pulses. We have applied this compatible system to study the surface and vibrational structures of riboflavin molecules (also known as vitamin B2). The SFG spectra of eight polarization combinations have non-vanishing signals. The signals with incoming s-polarized IR are relatively weaker than the signals with incoming p-polarized IR. Under the double resonant conditions, the SFG signals of the conjugated tricyclic ring are greatly enhanced. For the IIV-TPF-FWM spectra with incoming p-polarized IR, only the sspp and pppp polarization combinations have non-vanishing signals. The IIV-TPF-FWM spectra show a very strong peak at 1585 cm(-1) that is mainly dominated by the N(5)-C(4a) stretch. The method developed in this study will be helpful for researchers, either using a home-built or commercial (EKSPLA) SFG system, to obtain independent and complementary measurements for SFG spectroscopy and more detailed structural information of interfacial molecules.