Microscopic molecular optics theory of surface second harmonic generation and sum-frequency generation spectroscopy based on the discrete dipole lattice model

Theoretical investigation of vibrational energy transfer of water at the gas/liquid interface around 3400 cm-1

Using molecular dynamics simulations based on neural network potentials and a mixed quantum/classical approach, we theoretically explore vibrational energy transfer pathways for OH groups around 3400 cm-1 at interfaces. Our calculations show that intramolecular vibrational energy transfer has a time constant of 369.2 fs, aligning with experimental findings. The reorientation time is 1624 fs. The intermolecular vibrational energy transfer rate is slower due to weak couplings between water molecules. Our results suggest that intramolecular energy transfer is the main driver of vibrational energy transfer for OH groups around 3400 cm-1 at the gas/water interface.

Local field effects of quadrupole contributions on sum frequency generation spectroscopy

In the theory of condensed-phase spectroscopy, local field effect is of general importance to account for intermolecular electrostatic interactions. The present paper extends the microscopic treatment of local field effects on the sum frequency generation (SFG) spectroscopy to incorporate quadrupole interactions, since their roles have been increasingly recognized in the SFG spectroscopy. The extended theory involves some corrections to the conventional formulas of the nonlinear susceptibilities in both the interface and bulk regions, including the χIQB term. Fresnel transformations for the interface and bulk susceptibilities are rigorously applied, which implies inseparability of the interface and bulk signals in PSS and PPP cases. We examined the influence of the corrections with quantitative calculations of the susceptibilities, including dipolar and quadrupolar interactions.

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.

Surface populations as a model for the distance-dependence of the interfacial refractive index

Vibrational sum frequency spectra provide information about interfaces that is sensitive to the orientation of molecules, their electronic environment, and the local electric fields. Here, we use molecular dynamics simulations in order to study a surfactant, para-cyanophenol, at the air-water interface. The volume fractions of water and the organic surfactant are considered at various points over the nanometer-scale region in a Lorentz-Lorenz model. We find that the calculated ratios of nonlinear susceptibility tensor elements are in agreement with experimental data only when this depth profile was considered. We also use these data to evaluate the ratio of the C-N hyperpolarizability tensor elements in the interfacial region.

Second-Order Nonlinear Optics as an Orientation-Independent Probe of Molecular Environments at Interfaces

Measurement techniques that probe the second-order susceptibility, such as second-harmonic and sum-frequency generation, are recognized for their ability to study environments with broken centrosymmetry. As a result, they serve as reporters of molecules at surfaces because the second-order susceptibility is often zero in the adjacent bulk media. Although the signals measured in such experiments carry unique information about the interfacial environment, the challenge is to disentangle properties related to the electronic structure as they are wrapped up in the orientation distribution. Over the past 30 years, this challenge has been turned into an opportunity, as many studies have sought to learn about the arrangement of molecules at surfaces. Here we demonstrate that the flipped case is possible, where fundamental properties of the interfacial environment can be extracted in a manner that is completely independent of, and therefore oblivious to, the orientation distribution. Using p-cyanophenol adsorbed at the air-water interface as an example, we illustrate that the cyano group polarizability varies less along the direction of the C-N bond when at the surface than when the same molecules are in the bulk aqueous phase.

Sum-frequency vibrational spectroscopy of centrosymmetric molecule at interfaces

The centrosymmetric benzene molecule has zero first-order electric dipole hyperpolarizability, which results in no sum-frequency vibrational spectroscopy (SFVS) signal at interfaces, but it shows very strong SFVS experimentally. We perform a theoretical study on its SFVS, which is in good agreement with the experimental results. Its strong SFVS mainly comes from the interfacial electric quadrupole hyperpolarizability rather than the symmetry-breaking electric dipole, bulk electric quadrupole, and interfacial and bulk magnetic dipole hyperpolarizabilities, which provides a novel and completely unconventional point of view.

On the Fresnel factor correction of sum-frequency generation spectra of interfacial water

Insights into the microscopic structure of aqueous interfaces are essential for understanding the chemical and physical processes on the water surface, including chemical synthesis, atmospheric chemistry, and events in biomolecular systems. These aqueous interfaces have been probed by heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy. To obtain the molecular response from the measured HD-SFG spectra, one needs to correct the measured ssp spectra for local electromagnetic field effects at the interface due to a spatially varying dielectric function. This so-called Fresnel factor correction can change the inferred response substantially, and different ways of performing this correction lead to different conclusions about the interfacial water response. Here, we compare the simulated and experimental spectra at the air/water interface. We use three previously developed models to compare the experiment with theory: an advanced approach taking into account the detailed inhomogeneous interfacial dielectric profile and the Lorentz and slab models to approximate the interfacial dielectric function. Using the advanced model, we obtain an excellent quantitative agreement between theory and experiment, in both spectral shape and amplitude. Remarkably, we find that for the Fresnel factor correction of the ssp spectra, the Lorentz model for the interfacial dielectric function is equally accurate in the hydrogen (H)-bonded region of the response, while the slab model underestimates this response significantly. The Lorentz model, thus, provides a straightforward method to obtain the molecular response from the measured spectra of aqueous interfaces in the H-bonded region.

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.

Temporal and Chirp Effects of Laser Pulses on the Spectral Lineshape 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 lineshape 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 has not been widely adopted. The majority of SFG measurements so far still relies on the picosecond scanning SFG-VS (ps-SFG-VS) or the conventional broadband SFG-VS (BB-SFG-VS) with the spectral resolution around (mostly above) 10 cm-1, which also results in less ideal spectral lineshape in the SFG spectra due to the temporal and chirp effects of the laser pulses used in experiment. In this report, the temporal and the chirp effects of laser pulses with different profiles in the SFG experiment on the measured SFG-VS spectral lineshape are examined through spectral simulation. In addition, the experimental data of a classical model system, i.e., OTS (octadecyltrichlorosilane) monolayer on glass, obtained from the ps-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 lineshape 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 spectra.

Sum frequency generation as a proxy for ellipsometry: Not just a phase

Infrared refractive indices of organic materials are typically resolved through IR ellipsometry. This technique takes advantage of optical interference effects to solve the optical constants. These are the same effects that complicate the analysis of coherent spectroscopy experiments on thin films. Vibrational sum frequency generation is an interface-specific coherent spectroscopy that requires spectral modeling to account for optical interference effects to uncover interfacial molecular responses. Here, we explore the possibility of leveraging incident beam geometries and sample thicknesses to simultaneously obtain the molecular responses and refractive indices. Globally fitting a higher number of spectra with a single set of refractive indices increases the fidelity of the fitted parameters. Finally, we test our method on samples with a range of thicknesses and compare the results to those obtained by IR ellipsometry.

Sum-frequency vibrational spectroscopy of methanol at interfaces due to Fermi resonance

We present a theoretical method of studying sum-frequency vibrational spectroscopy for the CH3 group of methanol at interfaces due to Fermi resonance, which provides a novel and untraditional point of view with respect to traditional approaches.

Water Capture Mechanisms at Zeolitic Imidazolate Framework Interfaces

Water capture mechanisms of zeolitic imidazolate framework ZIF-90 are revealed by differentiating the water clustering and the center pore filling step, using vibrational sum-frequency generation spectroscopy (VSFG) at a one-micron spatial resolution and state-of-the-art molecular dynamics (MD) simulations. Through spectral line shape comparison between VSFG and IR spectra, the relative humidity dependence of VSFG intensity, and MD simulations, based on MB-pol, we found water clustering and center pore filling happen nearly simultaneously within each pore, with water filling the other pores sequentially. The integration of nonlinear optics with MD simulations provides critical mechanistic insights into the pore filling mechanism and suggests that the relative strength of the hydrogen bonds governs the water uptake mechanisms. This molecular-level detailed mechanism can inform the rational optimization of metal-organic frameworks for water harvesting.

Unveiling the Molecular Dynamics in a Living Cell to the Subcellular Organelle Level Using Second-Harmonic Generation Spectroscopy and Microscopy

Second-harmonic generation (SHG) microscopy has been proved to be a powerful method for investigating the structures of biomaterials. SHG spectra were also generally used to probe the adsorption and cross-membrane transport of molecules on lipid bilayers in situ and in real time. In this work, we applied SHG and two-photon fluorescence (TPF) spectra to investigate the dynamics of an amphiphilic ion with an SHG and TPF chromophore, D289 (4-(4-diethylaminostyry)-1-methyl-pyridinium iodide), on the surface of human chronic myelogenous leukemia (K562) cells and the subcellular structures inside the cells. The adsorption and cross-membrane transport of D289 into the cells and then into the organelles such as mitochondria were revealed. SHG images were also recorded and used to demonstrate their capability of probing molecular dynamics in organelles in K562 cells. This work demonstrated the first SHG investigation of the cross-membrane transport dynamics on the surface of subcellular organelles. It may also shed light on the differentiation of different types of subcellular structures in cells.

Herzberg-Teller Effect Predominates in Sum-Frequency Vibrational Spectroscopy of Limonene Chiral Liquids

We theoretically study the bulk sum-frequency vibrational spectroscopy of chiral liquids under the influence of the Franck-Condon, Herzberg-Teller, and nonadiabatic effects. With quantum chemistry computations we calculate the chiral spectra for the R-limonene molecule. When we compare the theoretical and experimental spectra, we find that the Herzberg-Teller effect under the Born-Oppenheimer approximation, instead of the nonadiabatic effect, predominates in the chiral spectra.

Non-parabolic potential dependence of optical second harmonic generation from the Si(111) electrode/electrolyte interface

We performed potential dependent second harmonic generation (SHG) measurements on the Si(111) electrolyte interface at different azimuthal angles and for different polarization combinations.

Molecular structure and adsorption of dimethyl sulfoxide at the air/aqueous solution interface probed by non-resonant second harmonic generation

In this study, non-resonant second harmonic generation (SHG) was used to investigate the molecular structure and adsorption of DMSO at the air/neat DMSO liquid and air/DMSO aqueous solution interfaces.

Theoretical investigation of quadrupole contributions to surface sum-frequency vibrational spectroscopy

We present a new method of calculating quadrupole contributions to surface sum-frequency vibrational spectroscopy.

Unified treatment and measurement of the spectral resolution and temporal effects in frequency-resolved sum-frequency generation vibrational spectroscopy (SFG-VS)

The lack of understanding of the temporal effects and the restricted ability to control experimental conditions in order to obtain intrinsic spectral lineshapes in surface sum-frequency generation vibrational spectroscopy (SFG-VS) have limited its applications in surface and interfacial studies. The emergence of high-resolution broadband sum-frequency generation vibrational spectroscopy (HR-BB-SFG-VS) with sub-wavenumber resolution [Velarde et al., J. Chem. Phys., 2011, 135, 241102] offers new opportunities for obtaining and understanding the spectral lineshapes and temporal effects in SFG-VS. Particularly, the high accuracy of the HR-BB-SFG-VS experimental lineshape provides detailed information on the complex coherent vibrational dynamics through direct spectral measurements. Here we present a unified formalism for the theoretical and experimental routes for obtaining an accurate lineshape of the SFG response. Then, we present a detailed analysis of a cholesterol monolayer at the air/water interface with higher and lower resolution SFG spectra along with their temporal response. With higher spectral resolution and accurate vibrational spectral lineshapes, it is shown that the parameters of the experimental SFG spectra can be used both to understand and to quantitatively reproduce the temporal effects in lower resolution SFG measurements. This perspective provides not only a unified picture but also a novel experimental approach to measuring and understanding the frequency-domain and time-domain SFG response of a complex molecular interface.

Theoretical study of sum-frequency vibrational spectroscopy on limonene surface

By combining molecule dynamics (MD) simulation and quantum chemistry computation, we calculate the surface sum-frequency vibrational spectroscopy (SFVS) of R-limonene molecules at the gas-liquid interface for SSP, PPP, and SPS polarization combinations. The distributions of the Euler angles are obtained using MD simulation, the ψ-distribution is between isotropic and Gaussian. Instead of the MD distributions, different analytical distributions such as the δ-function, Gaussian and isotropic distributions are applied to simulate surface SFVS. We find that different distributions significantly affect the absolute SFVS intensity and also influence on relative SFVS intensity, and the δ-function distribution should be used with caution when the orientation distribution is broad. Furthermore, the reason that the SPS signal is weak in reflected arrangement is discussed.

Throwing Salt into the Mix: Altering Interfacial Water Structure by Electrolyte Addition

Interfacial water commonly distinguishes itself from the bulk phase by adopting a polar, ordered structure. It is well-established that electrolytes can act to perturb this structure; however, the nature of this perturbation remains a topic of interest. In this study, surface- and structure-sensitive nonlinear vibrational spectroscopy is used to monitor electrolyte-induced changes in interfacial water structure. Solution ionic strength was varied over 5 orders of magnitude, and spectra were collected from two mineral surfaces (fused silica and calcium fluoride) and two polymer surfaces (polystyrene and poly(methyl methacrylate)). Analysis of the spectra reveals striking similarities and differences between these four aqueous interfaces; trends in overall intensity do not always follow changes in the spectral shape. Our results reveal the complex interplay between surface charge, ion adsorption, and hydrophobicity in determining interfacial water structure in the presence of dissolved ions.

Phase engineering in oxides by interfaces

Optical second harmonic generation and piezoresponse force microscopy are used to investigate manifestations of ordered states directly related to the presence of an oxide interface. Three examples, each with a very different scope, are reviewed in order to highlight the richness of interface-related phenomena in oxides. (i) The orbital states involved in the emergence of an interfacial conducting state in LaAlO(3)/SrTiO(3) heterostructures are investigated, which reveal a surprising decoupling of orbital and transport properties; (ii) the distribution of ferroelectric and antiferromagnetic domains in epitaxial films of the multiferroic hexagonal manganites is investigated, which reveals striking differences to the corresponding bulk crystals; and (iii) the distribution of trimerization-polarization domains in the hexagonal manganites is investigated, which reveals the presence of topologically protected domain walls with properties different from the bulk.