Broadband models and their consequences on line shape analysis in vibrational sum-frequency spectroscopy

Phase of the second-order susceptibility in vibrational sum frequency generation spectroscopy: Origins, utility, and measurement techniques

Vibrational sum frequency generation can provide valuable structural information at surfaces and buried interfaces. Relating the measured spectra to the complex-valued second-order susceptibility χ(2) is at the heart of the technique and a requisite step in nearly all subsequent analyses. The magnitude and phase of χ(2) as a function of frequency reveal important information about molecules and materials in regions where centrosymmetry is broken. In this tutorial-style perspective, the origins of the χ(2) phase are first described, followed by the utility of phase determination. Finally, some practical methods of phase extraction are discussed.

Study on improvement of copper sulfide acid soil properties and mechanism of metal ion fixation based on Fe-biochar composite

In this study, Fe modification of bamboo biochar (BC) with ferrate was used to construct a composite soil amendment based on K2FeO4-biochar (Fe-BC) system. Based on soil culture experiments, Fe-BC combined with organic-inorganic materials at the application levels of 3%, 5% and 10% to copper sulfide contaminated acid soil was studied. Adsorption kinetics experiment was used to investigate the adsorption capacity of Fe-modified biochar to heavy metal Cu. The results showed that the pH value of bamboo biochar could be increased by 1.12 units after K2FeO4 modification. Compared with the BC, the adsorption capacity of Cu2+ increased from 190.48 to 276.12 mg/g, which was mainly reflected in single-layer surface adsorption and chemisorption. Pore diffusion, electrostatic interaction and surface interaction are the possible mechanisms of Fe-BC interaction with Cu2+ ions. And the contents of Pb, Cu and Zn in soil leaching state decreased by 59.20%, 65.88% and 57.88%, respectively, at the 10% application level of Fe-BC. In general, the composite modifier based on ferrate and biochar has a positive effect on improving the characteristics of acidic soil in copper mining area.

Relating the phase in vibrational sum frequency spectroscopy and second harmonic generation with the maximum entropy method

Nonlinear optical methods, such as vibrational sum frequency generation (vSFG) and second harmonic generation (SHG), are powerful techniques to study elusive structures at charged buried interfaces. However, for the separation and determination of the Stern and diffuse layer spectra at these charged interfaces, complex vSFG spectra and, hence, the absolute phase need to be retrieved. The maximum entropy method is a useful tool for the retrieval of complex spectra from the intensity spectra; however, one caveat is that an understanding of the error phase is required. Here, for the first time, we provide a physically motivated understanding of the error phase. Determining the error phase from simulated spectra of oscillators with a spectral overlap, we show that for broadband vSFG spectra, such as for the silica/water interface, the diffuse and Stern layers' spectral overlap within the O-H stretching window results in a correlation between the error phase and the phase shift between the responses of these layers. This correlation makes the error phase sensitive to changes in Debye length from varying the ionic strength among other variations at the interface. Furthermore, the change in the magnitude of the error phase can be related to the absolute SHG phase, permitting the use of an error phase model that can utilize the SHG phase to predict the error phase and, hence, the complex vSFG spectra. Finally, we highlight limitations of this model for vSFG spectra with a poor overlap between the diffuse and Stern layer spectra (silica/HOD in D2O system).

Vibrational sum frequency spectroscopy of thin film interfaces

We describe a basic theoretical treatment of how film-substrate and substrate-environment (air, water, and solution) interfaces can be selectively probed by controlling the film thickness and beam angles in a visible-infrared sum frequency generation experiment. In this model, we also account for the unique interfacial environment that may have optical properties that differ from the adjacent bulk phases. We see that this affects components of the electric field that are perpendicular to the surface such as when p-polarized light is used. We then provide an example using the glass-polydimethylsiloxane-air system and model the fields at both surfaces of the polymer. This is followed by some practical considerations for setting up such experiments and some typical experimental results.

Water Structure in the Electrical Double Layer and the Contributions to the Total Interfacial Potential at Different Surface Charge Densities

The electric double layer governs the processes of all charged surfaces in aqueous solutions; however, elucidating the structure of the water molecules is challenging for even the most advanced spectroscopic techniques. Here, we present the individual Stern layer and diffuse layer OH stretching spectra at the silica/water interface in the presence of NaCl over a wide pH range using a combination of vibrational sum frequency generation spectroscopy, heterodyned second harmonic generation, and streaming potential measurements. We find that the Stern layer water molecules and diffuse layer water molecules respond differently to pH changes: unlike the diffuse layer, whose water molecules remain net-oriented in one direction, water molecules in the Stern layer flip their net orientation as the solution pH is reduced from basic to acidic. We obtain an experimental estimate of the non-Gouy-Chapman (Stern) potential contribution to the total potential drop across the insulator/electrolyte interface and discuss it in the context of dipolar, quadrupolar, and higher order potential contributions that vary with the observed changes in the net orientation of water in the Stern layer. Our findings show that a purely Gouy-Chapman (Stern) view is insufficient to accurately describe the electrical double layer of aqueous interfaces.

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.

Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability

Isolating the hydrogen-bonding structure of water immediately at the surface is challenging, even with surface-specific techniques like sum-frequency generation (SFG), because of the presence of aligned water further away in the diffuse layer. Here, we combine zeta potential and SFG intensity measurements with the maximum entropy method referenced to reported phase-sensitive SFG and second-harmonic generation results to deconvolute the SFG spectral contributions of the surface waters from those in the diffuse layer. Deconvolution reveals that at very low ionic strength, the surface water structure is similar to that of a neutral silica surface near the point-of-zero-charge with waters in different hydrogen-bonding environments oriented in opposite directions. This similarity suggests that the known metastability of silica colloids against aggregation under both conditions could arise from this distinct surface water structure. Upon the addition of salt, significant restructuring of water is observed, leading to a net decrease in order at the surface.

Phase-Sensitive Sum-Frequency Generation Measurements Using a Femtosecond Nonlinear Interferometer

Phase-sensitive sum-frequency spectroscopy is a unique tool to interrogate the vibrational structure of interfaces. A precise understanding of the interfacial structure often relies on accurately determining the phase of χ⁽²⁾, which has recently been demonstrated using a nonlinear interferometer in conjunction with a frequency-scanning picosecond laser system. Here, we implement nonlinear interferometry using a femtosecond laser system for broadband sum-frequency generation. The phase of the vibrational response from a self-assembled monolayer of octadecanethiol on gold is determined using the nonlinear femtosecond interferometer. The results are compared to those obtained using the more traditional heterodyne-detected phase measurements. Both methods give a similar phase spectrum and phase uncertainty. We also discuss the origin of the phase uncertainties and provide guidelines for further improvement.