Relative Order of Sulfuric Acid, Bisulfate, Hydronium, and Cations at the Air-Water Interface

Probing the adsorption configuration of methanol at a charged air/aqueous interface using nonlinear spectroscopy

Investigating the effects of electrolyte ions on the adsorption configuration of methanol at a charged interface is important for studying the interface structure of electrolyte solutions and the oxidation mechanism of methanol in fuel cells. This study uses sum frequency generation (SFG) and heterodyne-detected second harmonic generation (HD-SHG) to investigate the adsorption configuration of methanol at the air/aqueous interface of 0.1 M NaClO4 solution, 0.1 M HClO4 solution and pure water. The results elucidate that the ion effect in the electrolyte solution affects the interface's charged state and the methanol's adsorption conformation at the interface. The negatively charged surface of the 0.1 M NaClO4 solution and the positively charged surface of the 0.1 M HClO4 solution arise from the corresponding specific ionic effects of the electrolyte solution. The orientation angle of methyl with respect to the surface normal is 43.4° ± 0.1° at the 0.1 M NaClO4 solution surface and 21.5° ± 0.2° at the 0.1 M HClO4 solution surface. Examining these adsorption configurations in detail, we find that at the negatively charged surface the inclined orientation angle (43.4°) of methanol favors the hydroxymethyl production by breaking the C-H bond, while at the positively charged surface the upright orientation angle (21.5°) of methanol promotes the methoxy formation by breaking the O-H bond. These findings not only illuminate the intricate ion effects on small organic molecules but also contribute to a molecular-level comprehension of the oxidation mechanism of methanol at electrode interfaces.

Neural network potential energy surfaces and dipole moment surfaces for SO2(H2O) and SO2(H2O)2 complexes

Full-dimensional, ab initio-based many-body potential energy surfaces and dipole moment surfaces constructed using the neural network method for SO2(H2O)n (n = 1,2) complexes are reported. The database of the SO2 1-body PES, SO2(H2O) 2-body PES and SO2(H2O)2 3-body PES consists of 11 952, 79 882 and 84 159 ab initio energies, respectively. All 1-body energies were calculated at the CCSD(T)/CBS(AVTZ:AVQZ) level and all 2,3-body energies were calculated at the DSD-PBEP86/AVTZ level. The database of DMSs is the same as that of PESs and all dipole moments were calculated at the MP2/AVTZ level. Harmonic frequencies and dissociation energies of SO2(H2O) and SO2(H2O)2 were calculated on these PESs and compared with ab initio results to examine the fidelity of these PESs.

The Three-Phase Contact Potential Difference Modulates the Water Surface Charge

The surface charge of an open water surface is crucial for solvation phenomena and interfacial processes in aqueous systems. However, the magnitude of the charge is controversial, and the physical mechanism of charging remains incompletely understood. Here we identify a previously overlooked physical mechanism determining the surface charge of water. Using accurate charge measurements of water microdrops, we demonstrate that the water surface charge originates from the electrostatic effects in the contact line vicinity of three phases, one of which is water. Our experiments, theory, and simulations provide evidence that a junction of two aqueous interfaces (e.g., liquid-solid and liquid-air) develops a pH-dependent contact potential difference Δϕ due to the longitudinal charge redistribution between two contacting interfaces. This universal static charging mechanism may have implications for the origin of electrical potentials in biological, nanofluidic, and electrochemical systems and helps to predict and control the surface charge of water in various experimental environments.

Imaging of pH distribution inside individual microdroplet by stimulated Raman microscopy

Aerosol microdroplets as microreactors for many important atmospheric reactions are ubiquitous in the atmosphere. pH largely regulates the chemical processes within them; however, how pH and chemical species spatially distribute within an atmospheric microdroplet is still under intense debate. The challenge is to measure pH distribution within a tiny volume without affecting the chemical species distribution. We demonstrate a method based on stimulated Raman scattering microscopy to visualize the three-dimensional pH distribution inside single microdroplets of varying sizes. We find that the surface of all microdroplets is more acidic, and a monotonic trend of pH decreasing is observed in the 2.9-μm aerosol microdroplet from center to edge, which is well supported by molecular dynamics simulation. However, bigger cloud microdroplet differs from small aerosol for pH distribution. This size-dependent pH distribution in microdroplets can be related to the surface-to-volume ratio. This work presents noncontact measurement and chemical imaging of pH distribution in microdroplets, filling the gap in our understanding of spatial pH in atmospheric aerosol.

Chemical Implications of Rapid Reactive Absorption of I2O4 at the Air-Water Interface

Marine aerosol formation involving iodine-bearing species significantly affects the global climate and radiation balance. Although recent studies outline the critical role of iodine oxide in nucleation, much less is known about its contribution to aerosol growth. This paper presents molecular-level evidence that the air-water interfacial reaction of I₂O₄ mediated by potent atmospheric chemicals, such as sulfuric acid (H₂SO₄) and amines [e.g., dimethylamine (DMA) and trimethylamine (TMA)], can occur rapidly on a picosecond time scale by Born-Oppenheimer molecular dynamics simulations. The interfacial water bridges the reactants while facilitating the DMA-mediated proton transfer and stabilizing the ionic products of H₂SO₄-involved reactions. The identified heterogeneous mechanisms exhibit the dual contribution to aerosol growth: (i) the ionic products (e.g., IO₃^-, DMAH⁺, TMAH⁺, and HSO₄^-) formed by reactive adsorption possess less volatility than the reactants and (ii) these ions, such as alkylammonium salts (e.g., DMAH⁺), are also highly hydrophilic, further facilitating hygroscopic growth. This investigation enhances not only our understanding of heterogeneous iodine chemistry but also the impact of iodine oxide on aerosol growth. Also, these findings can bridge the gap between the abundance of I₂O₄ in the laboratory and its absence in field-collected aerosols and provide an explanation for the missing source of IO₃^-, HSO₄^-, and DMAH⁺ in marine aerosols.

Enhanced uptake of methacrolein at the acidic nanoparticle interface: Adsorption, heterogeneous reaction and impact for the secondary organic aerosol formation

Considering the moderate acidity of aerosols, the formation of secondary organic aerosols (SOA) through acid-catalyzed heterogeneous reactions has become a recent concern. However, the detailed information on the multiphase chemistry of organic compounds adsorbed onto acidic aerosols remains uncertain. In this work, we investigated the multiphase chemical processes between methacrolein (MACR) and sulfuric acid (SA) and their relationship with SOA formation. Results show that the aqueous nanoparticle interface, especially when it is an acidic nanoparticle interface, is a perfect area to adsorb and accommodate MACR. The occurrence percentage of MACR on the interface is more than 70%. With the increase of SA concentration, the first solvation shell changed from only water to the mixture of SA and water, which facilitates the heterogeneous hydration reaction of MACR. Compared with the neutral nanoparticle interface, the acidic nanoparticle interface exhibits a better ability to uptake and accommodate gaseous carbonyl species. Moreover, SA can catalyze the hydration reaction of MACR inside the aqueous media, and the resulting oligomers contribute to the formation and growth of SOA. The hydration reaction indirectly promotes the continuous adsorption of MACR at the acidic nanoparticle interface. The rate constant shows a positive altitude dependence, and acid-catalyzed reactions have an important impact on environmental chemistry, such as cloud SOA formation, within the range of about 2-6 km. This study reports a complete description of the heterogeneous interactions between unsaturated carbonyl species and acidic nanoparticles by using molecular dynamics and quantum chemistry methods, aiming to provide some insights for the further study on heterogeneous chemistry and its role in the formation of tropospheric SOA.

The enhanced dissociation and associated surface structure of the anesthetic propofol at the water interface: vibrational sum frequency generation study

Propofol, the most administered drug for general anesthesia, affects the acid-base equilibrium at the interfacial region of arterial blood. Hence, the structure of propofol at the water interface under different pH conditions has been measured using the surface-selective vibrational sum frequency generation (VSFG) technique to understand the hydration as well as the dissociation of propofol at the water interface. Propofol remains in its neutral form at pH ≤ 5.8 in which the OH group of propofol forms a hydrogen bond with interfacial water molecules, where a few interfacial water molecules also interact with the π electron density of propofol. By contrast, propofol prefers to be in the deprotonated state at pH ≥ 7, due to which the surface of water becomes negatively charged and hence the interfacial water becomes oriented and the intensity of the OH stretch of water is enhanced. The pK_a of propofol at the water interface is ∼three units lower than in the bulk medium indicating that the dissociation of propofol is notably enhanced at the water interface. These VSFG studies suggest that, unlike the bulk, propofol prefers to be in the charged state at the water interface under physiological conditions, which may be important in understanding its diffusion and acid-base equilibrium in the interfacial arterial blood region.

Electrification of water interface

The surface charge of a water interface determines many fundamental processes in physical chemistry and interface science, and it has been intensively studied for over a hundred years. We summarize experimental methods to characterize the surface charge densities developed so far: electrokinetics, double-layer force measurements, potentiometric titration, surface-sensitive nonlinear spectroscopy, and surface-sensitive mass spectrometry. Then, we elucidate physical ion adsorption and chemical electrification as examples of electrification mechanisms. In the end, novel effects on surface electrification are discussed in detail. We believe that this clear overview of state of the art in a charged water interface will surely help the fundamental progress of physics and chemistry at interfaces in the future.

Ion Pairing Mediates Molecular Organization Across Liquid/Liquid Interfaces

Liquid/liquid interfaces play a central role in scientific fields ranging from nanomaterial synthesis and soft matter electronics to nuclear waste remediation and chemical separations. This diversity of functions arises from an interface's ability to respond to changing conditions in its neighboring bulk phases. Understanding what drives this interfacial flexibility can provide novel avenues for designing new functional interfaces. However, limiting this progress is an inadequate understanding of the subtle intermolecular and interphase interactions taking place at the molecular level. Here, we use surface-specific vibrational sum frequency generation spectroscopy combined with atomistic molecular dynamics simulations to investigate the self-assembly and structure of model ionic oligomers consisting of an oligodimethylsiloxane (ODMS) tail covalently attached to a positively charged methyl imidazolium (MIM⁺) head group at buried oil/aqueous interfaces. We show how the presence of seemingly innocuous salts can impart dramatic changes to the ODMS tail conformations in the oil phase via specific ion effects and ion-pairing interactions taking place in the aqueous phase. These specific ion interactions are shown to drive enhanced amphiphile adsorption, induce morphological changes, and disrupt emergent hydrogen-bonding structures at the interface. Tuning these interactions allows for independent control over the oligomer structure in the oil phase versus interfacial population changes and represents key mechanistic insight that is needed to control chemical reactions at liquid/liquid interfaces.

Barrierless HONO and HOS(O)2-NO2 Formation via NH3-Promoted Oxidation of SO2 by NO2

In the troposphere, the knowledge about nitrous acid (HONO) sources is incomplete. The missing source of sulfate and fine particles cannot be explained during haze events. Air quality models cannot predict high levels of secondary fine-particle pollution. Despite extensive studies, one challenging issue in atmospheric chemistry is identifying the source of HONO. Here, we present direct ab initio molecular dynamics simulation evidence and typical air pollution events of the formation of gaseous HONO, nitrogen dioxide/hydrogen sulfite (HOS(O)2-NO₂ or NO₂-HSO₃) from nitrogen dioxide (NO₂), sulfur dioxide (SO₂), water (H₂O), and ammonia (NH₃) molecules in a proportion of 2:1:3:3. The reactions show a new mechanism for the formation of HONO and NO₂-HSO₃ in the troposphere, especially when the concentration of NO₂, SO₂, H₂O, and NH₃ is high (e.g., 2:1:3:3 or higher) in the air. Contrary to the proportion NO₂, SO₂, H₂O, and NH₃ equaling to 1:1:3:1 and 1:1:3:2, the proportion (2:1:3:3) enables barrierless reactions and weak interactions between molecules via the formation of HONO, NO₂-HSO₃, and NH₃/H₂O. In addition, field observations are carried out, and the measured data are summarized. Correlation analysis supported the conversion of NO₂ to HONO during observational studies. The weak interactions promote proton transfer, resulting in the generation of HONO, NO₂-HSO₃, and NH₃/H₂O pairs.

Direct Quantification of the Effect of Ammonium on Aerosol Droplet pH

Ammonium is an important atmospheric constituent that dictates many environmental processes. The impact of the ammonium ion concentration on 10-50 μm aerosol droplet pH was quantified using pH nanoprobes and surface-enhanced Raman spectroscopy (SERS). Sample solutions were prepared by mixing 1 M ammonium sulfate (AS), ammonium nitrate (AN), sodium sulfate (SS), or sodium nitrate (SN) solutions with 1 M phosphate buffer (PB) at different volume ratios. Stable pH values were measured for pure PB, AS, and AN droplets at different concentrations. The centroid pH of 1 M PB droplets was ∼11, but when PB was systematically replaced with ammonium (AS- or AN-PB), the centroid pH within the droplets decreased from ≈11 to 5.5. Such a decrease was not observed in sodium (SS- or SN-PB) droplets, and no pH differences were observed between sulfate and nitrate salts. Ammonia partitioning to the gas phase in ammonium-containing droplets was evaluated to be negligible. Raman sulfate peak (∼980 cm^-1) intensity measurements and surface tension measurements were conducted to investigate changes in ion distribution. The pH difference between ammonium-containing droplets and ammonium-free droplets is attributed to the alteration of the ion distribution in the presence of ammonium.

Water Promotes the Oxidation of SO2 by O2 over Carbonaceous Aerosols

Severe haze episodes typically occur with concurrent high relative humidity. Here, the vital role of water in promoting the oxidation of SO₂ by O₂ on carbonaceous soot surfaces was identified at the atomic level by first-principles calculations. Water molecules can dissociate into surface hydroxyl groups through a self-catalyzed process under ambient conditions. The surface hydroxyl groups, acting as facilitators, can significantly accelerate the conversion of SO₂ to SO₃ (precursor of particulate sulfate) over soot aerosols by reducing the reaction barriers. Specifically, the hydroxyl groups activate the reactants and stabilize the transition states and products through hydrogen-bonding interactions, making the reactions both thermodynamically and kinetically more favorable at room temperature. The findings indicate that atmospheric humidity plays an important role in enhancing the atmospheric oxidation capacity, thus exacerbating SO₂ oxidation and severe haze development. Also, this study unravels a mechanism of surface hydroxyl-assisted O₂ and H₂O dissociation over metal-free carbocatalysts under normal conditions.

Experimental Depth Profiles of Surfactants, Ions, and Solvent at the Angstrom Scale: Studies of Cationic and Anionic Surfactants and Their Salting Out

Neutral impact ion scattering spectroscopy (NICISS) is used to measure the depth profiles of ionic surfactants, counterions, and solvent molecules on the angstrom scale. The chosen surfactants are 0.010 m tetrahexylammonium bromide (THA⁺/Br^-) and 0.0050 m sodium dodecyl sulfate (Na⁺/DS^-) in the absence and presence of 0.30 m NaBr in liquid glycerol. NICISS determines the depth profiles of the elements C, O, Na, S, and Br through the loss in energy of 5 keV He atoms that travel into and out of the liquid, which is then converted into depth. In the absence of NaBr, we find that THA⁺ and its Br^- counterion segregate together because of charge attraction, forming a narrow double layer that is 10 Å wide and 150 times more concentrated than in the bulk. With the addition of NaBr, THA⁺ is "salted out" to the surface, increasing the interfacial Br^- concentration by 3-fold and spreading the anions over a ∼30 Å depth. Added NaBr similarly increases the interfacial concentration of DS^- ions and broadens their positions. Conversely, the dissolved Br^- ions are significantly depleted over a depth of 0-40 Å from the surface because of charge repulsion from DS^- ions within the interfacial region. These different interfacial Br^- propensities correlate with previously measured gas-liquid reactivities: gaseous Cl₂ readily reacts with Br^- ions in the presence of THA⁺ but drops 70-fold in the presence of DS^-, demonstrating that surfactant headgroup charge controls the reactivity of Br^- through changes in its depth profile.

Affinity of Hydrated Protons at Intrinsic Water/Vapor Interface Revealed by Ion-Induced Water Alignment

Protons at the water/vapor interface are relevant for atmospheric and environmental processes, yet characterizing their surface affinity on the quantitative level is still challenging. Here we utilize phase-sensitive sum-frequency vibrational spectroscopy to quantify the surface density of protons (or their hydronium form) at the intrinsic water/vapor interface through inspecting the surface-field-induced alignment of water molecules in the electrical double layer of ions. With hydrogen halides in water, the surface adsorption of protons is found to be independent of specific proton-halide anion interactions and to follow a constant adsorption free energy, ΔG ≈ -3.76 (±0.79) kJ/mol, corresponding to a partitioning coefficient of the surface with respect to bulk water by 3.3∼6.2, for bulk ion concentrations up to 0.3 M. Our spectroscopic study not only is of importance in atmospheric chemistry but also offers a microscopic-level basis to develop advanced quantum-mechanical models for molecular simulations.

Understanding Hygroscopic Nucleation of Sulfate Aerosols: Combination of Molecular Dynamics Simulation with Classical Nucleation Theory

We present a combined molecular dynamics (MD) and classical nucleation theory (CNT) approach to address many issues regarding the nucleation of inorganic aerosols. By taking parameters from MD simulations, we find the CNT predicts fairly reasonable free-energy profiles for the hygroscopic nucleation of aerosols. Moreover, we find that the ionization of sulfates can play a key role in stabilizing aqueous clusters and that both the size of the critical nucleus and the nucleation barrier can be significantly lowered by the H₂SO₄ and NH₄HSO₄, whereas the effect of NH₃ on nucleation is negligible. NH₄HSO₄ provides stronger enhancement effect to aerosol formation than H₂SO₄. In view of the consistency between the theoretical prediction and experimental observation, the combination of MD simulation and CNT appears to be a valuable approach to gain deeper understanding of how aerosol nucleation is affected by different chemical species.

The surface affinity of cations depends on both the cations and the nature of the surface

Specific ion effects at interfaces are important for a variety of thermodynamic properties of electrolyte solutions, like surface tension and the phase behavior of surfactants. We report the relative surface affinity of Na⁺ and D₃O⁺ at both the D₂O-air and the sodium dodecyl sulfate (surfactant)-covered D₂O surface by studying the alignment of interfacial D₂O, using vibrational sum frequency generation spectroscopy. The surface propensity of ions is found to be a function of both the nature of the ion and the nature of the surface. Specifically, for the charged, surfactant-covered interface, Na⁺ has a higher affinity than D₃O⁺. In contrast, D₃O⁺ has a higher affinity than Na⁺ at the air-D₂O interface. The relative surface affinity of cations thus depends on both details of the cation and the type of interface.

Competitive Adsorption of Ions at the Oil-Water Interface: A Possible Mechanism Underlying the Separation Selectivity for Liquid-Liquid Solvent Extraction

Adsorption, especially competitive adsorption of ions at the interfaces, governs a wealth of physicochemical processes. Understanding the mechanism behind these interfacial behaviors is crucial for developing novel strategies to intensify reactions or transfer processes. Herein, as an example, we found that in the case of liquid-liquid transport of V(V) and Cr(VI) ions, the competitive adsorption of V(V) and Cr(VI) ions against coexisting SO₄^2- ions at the oil-water interface exhibits a significant impact on the selective separation behaviors of V(V) and Cr(VI) ions. The transport of Cr(VI) ions would be hindered by adding Na₂SO₄ into the aqueous solutions because of the competitive adsorption of SO₄^2- ions at the interface being stronger than that of Cr(VI) ions, whereas the transport of V(V) ions would not be affected because of the stronger affinity of V(V) ions to the interfaces compared to that of SO₄^2- ions. The present work provides new inspirations for developing efficient strategies to improve the separation efficiency of target ions with similar physic-chemical properties by regulating their adsorption behaviors at the interface. It is beneficial to get a deeper understanding into the microscopic nature of competitive adsorption behaviors of ions at interfaces from the interface-molecular level.

Spectroscopic BIL-SFG Invariance Hides the Chaotropic Effect of Protons at the Air-Water Interface

The knowledge of the water structure at the interface with the air in acidic pH conditions is of utmost importance for chemistry in the atmosphere. We shed light on the acidic air-water (AW) interfacial structure by DFT-MD simulations of the interface containing one hydronium ion coupled with theoretical SFG (Sum Frequency Generation) spectroscopy. The interpretation of SFG spectra at charged interfaces requires a deconvolution of the signal into BIL (Binding Interfacial Layer) and DL (Diffuse Layer) SFG contributions, which is achieved here, and hence reveals that even though H 3 O + has a chaotropic effect on the BIL water structure (by weakening the 2D-HBond-Network observed at the neat air-water interface) it has no direct probing in SFG spectroscopy. The changes observed experimentally in the SFG of the acidic AW interface from the SFG at the neat AW are shown here to be solely due to the DL-SFG contribution to the spectroscopy. Such BIL-SFG and DL-SFG deconvolution rationalizes the experimental SFG data in the literature, while the hydronium chaotropic effect on the water 2D-HBond-Network in the BIL can be put in perspective of the decrease in surface tension at acidic AW interfaces.

Negative ion photoelectron spectra of ISO3-, IS2O3-, and IS2O4- intermediates formed in interfacial reactions of ozone and iodide/sulfite aqueous microdroplets

Three short-lived, anionic intermediates, ISO₃^-, IS₂O₃^-, and IS₂O₄^-, are detected during reactions between ozone and aqueous iodine/sulfur oxide microdroplets. These species may play an important role in ozone-driven inorganic aerosol formation; however their chemical properties remain largely unknown. This is the issue addressed in this work using negative ion photoelectron spectroscopy (NIPES) and ab initio modeling. The NIPE spectra reveal that all of the three anionic species are characterized by high adiabatic detachment energies (ADEs) - 4.62 ± 0.10, 4.52 ± 0.10, and 4.60 ± 0.10 eV for ISO₃^-, IS₂O₃^-, and IS₂O₄^-, respectively. Vibrational progressions with frequencies assigned to the S-O symmetric stretching modes are discernable in the ground state transition features. Density functional theory calculations show the presence of several low-lying isomers involving different bonding scenarios. Further analysis based on high level CCSD(T) calculations reveal that the lowest energy structures are characterized by the formation of I-S and S-S bonds and can be structurally viewed as SO₃ linked with I, IS, and ISO for ISO₃^-, IS₂O₃^-, and IS₂O₄^-, respectively. The calculated ADEs and vertical detachment energies are in excellent agreement with the experimental results, further supporting the identified minimum energy structures. The obtained intrinsic molecular properties of these anionic intermediates and neutral radicals should be useful to help understand their photochemical reactions in the atmosphere.

Experimentally quantifying anion polarizability at the air/water interface

The adsorption of large, polarizable anions from aqueous solution on the air/water interface controls important atmospheric chemistry and is thought to resemble anion adsorption at hydrophobic interfaces generally. While the favourability of adsorption of such ions is clear, quantifying adsorption thermodynamics has proven challenging because it requires accurate description of the structure of the anion and its solvation shell at the interface. In principle anion polarizability offers a structural window, but to the best of our knowledge there has so far been no experimental technique that allowed its characterization with interfacial specificity. Here, we meet this challenge using interface-specific vibrational spectroscopy of Cl–O vibrations of the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{ClO}}_4^ -$$\end{document}ClO4- anion at the air/water interface and report that the interface breaks the symmetry of the anion, the anisotropy of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{ClO}}_4^ -$$\end{document}ClO4-’s polarizability tensor is more than two times larger than in bulk water and concentration dependent, and concentration-dependent polarizability changes are consistent with correlated changes in surface tension.

Understanding anion-specific interactions with hydrophobic interfaces is challenging due to an absence of local structural probes. Here, the authors experimentally quantify the anisotropy of perchlorate’s polarizability at the air/water interface, a window into anion and solvation shell structure.

The structure of a lanthanide complex at an extractant/water interface studied using heterodyne-detected vibrational sum frequency generation

Eu³⁺ at an extractant/water interface is bound to extractants from the upper side and to water molecules from the lower side, and forms a unique interfacial complex.

Structure at the air/water interface in the presence of phenol: a study using heterodyne-detected vibrational sum frequency generation and molecular dynamics simulation

A simple, neutral organic molecule, phenol, forms a specific hydrogen-bonding structure with water at the air/water interface.

Frequency-domain nonlinear regression algorithm for spectral analysis of broadband SFG spectroscopy

The resonant spectral bands of the broadband sum frequency generation (BB-SFG) spectra are often distorted by the nonresonant portion and the lineshapes of the laser pulses. Frequency domain nonlinear regression (FDNLR) algorithm was proposed to retrieve the first-order polarization induced by the infrared pulse and to improve the analysis of SFG spectra through simultaneous fitting of a series of time-resolved BB-SFG spectra. The principle of FDNLR was presented, and the validity and reliability were tested by the analysis of the virtual and measured SFG spectra. The relative phase, dephasing time, and lineshapes of the resonant vibrational SFG bands can be retrieved without any preset assumptions about the SFG bands and the incident laser pulses.