Doubly resonant sum frequency spectroscopy of mixed photochromic isomers on surfaces reveals conformation-specific vibronic effects

Vibronic coupling of Rhodamine 6G molecules studied by doubly resonant sum frequency generation spectroscopy with narrowband infrared and broadband visible

Doubly resonant sum frequency generation (DR-SFG) serves as a potent characteristic technique for probing the electronic spectra and vibronic coupling of molecules on surfaces. In this study, we successfully developed a novel infrared (IR)-white light (WL) DR-SFG spectroscopy based on narrowband IR and tunable broadband WL. This novel method was employed to explore the excitation spectrum and vibronic couplings of sub-monolayer Rhodamine 6G molecules. Our findings elucidate that the xanthene skeleton vibrational modes exhibit strong coupling with the S0-S1 electronic transition. Notably, we observed not only the 0-0 transition of the S0-S1 electronic continuum but also the 0-1 transition, a first time observation in the realm of DR-SFG spectroscopy. This advanced DR-SFG spectroscopy methodology facilitates a more sensitive examination of electronic spectra and the coupling between electronic transitions and vibrational modes, heralding a significant advancement in the understanding of molecular interactions on surfaces.

DFT-Based Calculation of Molecular Hyperpolarizability and SFG Intensity of Symmetric and Asymmetric Stretch Modes of Alkyl Groups

Vibrational sum frequency generation (SFG) spectroscopy has been extensively used for obtaining structural information of molecular functional groups at two-dimensional (2D) interfaces buried in the gas or liquid medium. Although the SFG experiment can be done elegantly, interpreting the measured intensity in terms of molecular orientation with respect to the lab coordinate is quite complicated. One of the main reasons is the difficulty of determining the hyperpolarizability tensors of even simple molecules that govern their SFG responses. The single-bond polarizability derivative model has been proposed to estimate the relative magnitude of SFG-active hyperpolarizability by assuming that the perturbation associated to each vibration is negligible. In this study, density functional theory was used to calculate the polarizability and dipole derivative tensors of the CH3 stretch mode of CH3I, CH3CH2I, CH3OH, and CH3CH2OH. Then, the hyperpolarizability tensors of symmetric and asymmetric vibration modes were calculated considering the Boltzmann distribution of representative conformers, which allowed us to theoretically calculate their SFG intensities at all polarization combinations as a function of the tilt angle of the CH3 group with respect to the surface normal direction. Then, the ratios of the calculated SFG intensities for the CH3 symmetric and asymmetric stretch peaks used in experimental studies for the CH3 tilt angle determination were compared. This comparison clearly showed that the effect of vibrational coupling among neighboring functional groups is significant and cannot be assumed to be negligible. This study presents new parameters that can be used in determining the average tilt angle of the CH3 group at the 2D interface with SFG measurements as well as limitations of the method.

All-experimental analysis of doubly resonant sum-frequency generation spectra for Franck–Condon and Herzberg–Teller vibronic modes

The transform technique applied to the analysis of doubly resonant sum-frequency generation (DR-SFG) spectra is extended to include Herzberg–Teller (HT) vibronic modes. The experimentally measured overlap spectral function generates all the energy resonant amplitudes of the DR-SFG excitation function for both Franck–Condon (FC) and HT modes. When FC modes dominate the DR-SFG spectra, a methodology is provided to perform efficient curve fitting and orientation analysis in order to extract FC activities of the various vibration modes from experimental spectra with the help of a molecular model. Determination of the FC or HT natures of the vibration modes from DR-SFG data is also shown to be possible through their visible line shapes with an appropriate choice of polarizations. As an example, experimental DR-SFG data suggest that a known HT-active mode in the vibronic structure of Rhodamine 6G monomers exhibits a FC behavior in molecular aggregates.

Considerations in upconversion: A practical guide to sum-frequency generation spectrometer design and implementation

In this tutorial review, we discuss how the choice of upconversion pulse shape in broadband vibrational sum-frequency generation (SFG) spectrometer design impacts the chemical or physical insights one can obtain from a set of measurements. A time-domain picture of a vibrational coherence being mapped by a second optical field is described and the implications of how this mapping, or upconversion process, takes place are given in the context of several popular and emerging approaches found in the literature. Emphasis is placed on broadband frequency-domain measurements, where the choice of upconversion pulse enhances or limits the information contained in the SFG spectrum. We conclude with an outline for a flexible approach to SFG upconversion using pulse-shaping methods and a simple guide to design and optimize the associated instrumentation.

Doubly Resonant Sum-Frequency Vibrational Spectroscopy of 1,1'-Bi-2-naphthol Chiral Solutions Due to the Nonadiabatic Effect

Second-order nonlinear spectroscopy is a powerful tool in exploring significant physical and chemical characteristics at various interfaces and on chiral systems. We present a method of computing the nonadiabatic couplings between the different excited electronic states with time-dependent density functional theory and then study doubly resonant sum-frequency vibrational spectroscopy (SFVS) of chiral solutions due to the nonadiabatic, Franck-Condon, and Herzberg-Teller (HT) effects. The calculations for R-1,1'-bi-2-naphthol show that the theoretical spectra agree with experiment, and the nonadiabatic corrections are comparable with the HT terms or even larger for some vibrational modes, which is different from the mechanism of SFVS off electronic resonance. Doubly resonant SFVS may be a useful method of studying the nonradiative transition and nonadiabatic effect between the excited electronic states.

Two-Colour Sum-Frequency Generation Spectroscopy Coupled to Plasmonics with the CLIO Free Electron Laser

Nonlinear plasmonics requires the use of high-intensity laser sources in the visible and near/mid-infrared spectral ranges to characterise the potential enhancement of the vibrational fingerprint of chemically functionalised nanostructured interfaces aimed at improving the molecular detection threshold in nanosensors. We used Two-Colour Sum-Frequency Generation (2C-SFG) nonlinear optical spectroscopy coupled to the European CLIO Free Electron Laser in order to highlight an energy transfer in organic and inorganic interfaces built on a silicon substrate. We evidence that a molecular pollutant, such as thiophenol molecules adsorbed on small gold metal nanospheres grafted on silicon, was detected at the monolayer scale in the 10 µm infrared spectral range, with increasing SFG intensity of three specific phenyl ring vibration modes reaching two magnitude orders from blue to green–yellow excitation wavelengths. This observation is related to a strong plasmonic coupling to the thiophenol molecules vibrations. The high level of gold nanospheres aggregation on the substrate allows us to dramatically increase the presence of hotspots, revealing collective plasmon modes based on strong local electric fields between the gold nanoparticles packed in close contact on the substrate. This configuration favors detection of Raman active vibration modes, for which 2C-SFG spectroscopy is particularly efficient in this unusual infrared spectral range.

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.

Two-dimensional electronic-vibrational sum frequency spectroscopy for interactions of electronic and nuclear motions at interfaces

Interactions of electronic and vibrational degrees of freedom are essential for understanding excited-states relaxation pathways of molecular systems at interfaces and surfaces. Here, we present the development of interface-specific two-dimensional electronic-vibrational sum frequency generation (2D-EVSFG) spectroscopy for electronic-vibrational couplings for excited states at interfaces and surfaces. We demonstrate this 2D-EVSFG technique by investigating photoexcited interface-active (E)-4-((4-(dihexylamino) phenyl)diazinyl)-1-methylpyridin-1- lum (AP3) molecules at the air-water interface as an example. Our 2D-EVSFG experiments show strong vibronic couplings of interfacial AP3 molecules upon photoexcitation and subsequent relaxation of a locally excited (LE) state. Time-dependent 2D-EVSFG experiments indicate that the relaxation of the LE state, S ₂, is strongly coupled with two high-frequency modes of 1,529.1 and 1,568.1 cm^-1 Quantum chemistry calculations further verify that the strong vibronic couplings of the two vibrations promote the transition from the S ₂ state to the lower excited state S ₁ We believe that this development of 2D-EVSFG opens up an avenue of understanding excited-state dynamics related to interfaces and surfaces.

All-experimental analysis of doubly resonant sum-frequency generation spectra: Application to aggregated rhodamine films

A new method is proposed to analyze Doubly Resonant infrared-visible Sum-Frequency Generation (DR-SFG) spectra. Based on the transform technique, this approach is free from assumptions about vibronic modes, energies, or line widths and accurately captures through the overlap spectral function all required aspects of the vibronic structure from simple experimental linear absorption spectra. Details and implementation of the method are provided along with three examples treating rhodamine thin films about one monolayer thick. The technique leads to a perfect agreement between experiment and simulations of the visible DR-SFG line shapes, even in the case of complex intermolecular interactions resulting from J-aggregated chromophores in heterogeneous films. For films with mixed H- and J-aggregates, separation of their responses shows that the J-aggregate DR-SFG response is dominant. Our analysis also accounts for the unexplained results published in the early times of DR-SFG experiments.

Development of interface-/surface-specific two-dimensional electronic spectroscopy

Structures, kinetics, and chemical reactivities at interfaces and surfaces are key to understanding many of the fundamental scientific problems related to chemical, material, biological, and physical systems. These steady-state and dynamical properties at interfaces and surfaces require even-order techniques with time-resolution and spectral-resolution. Here, we develop fourth-order interface-/surface-specific two-dimensional electronic spectroscopy, including both two-dimensional electronic sum frequency generation (2D-ESFG) spectroscopy and two-dimensional electronic second harmonic generation (2D-ESHG) spectroscopy, for structural and dynamics studies of interfaces and surfaces. The 2D-ESFG and 2D-ESHG techniques were based on a unique laser source of broadband short-wave IR from 1200 nm to 2200 nm from a home-built optical parametric amplifier. With the broadband short-wave IR source, surface spectra cover most of the visible light region from 480 nm to 760 nm. A translating wedge-based identical pulses encoding system (TWINs) was introduced to generate a phase-locked pulse pair for coherent excitation in the 2D-ESFG and 2D-ESHG. As an example, we demonstrated surface dark states and their interactions of the surface states at p-type GaAs (001) surfaces with the 2D-ESFG and 2D-ESHG techniques. These newly developed time-resolved and interface-/surface-specific 2D spectroscopies would bring new information for structure and dynamics at interfaces and surfaces in the fields of the environment, materials, catalysis, and biology.

A Unified Mathematical Formalism for First to Third Order Dielectric Response of Matter: Application to Surface-Specific Two-Colour Vibrational Optical Spectroscopy

To take advantage of the singular properties of matter, as well as to characterize it, we need to interact with it. The role of optical spectroscopies is to enable us to demonstrate the existence of physical objects by observing their response to light excitation. The ability of spectroscopy to reveal the structure and properties of matter then relies on mathematical functions called optical (or dielectric) response functions. Technically, these are tensor Green’s functions, and not scalar functions. The complexity of this tensor formalism sometimes leads to confusion within some articles and books. Here, we do clarify this formalism by introducing the physical foundations of linear and non-linear spectroscopies as simple and rigorous as possible. We dwell on both the mathematical and experimental aspects, examining extinction, infrared, Raman and sum-frequency generation spectroscopies. In this review, we thus give a personal presentation with the aim of offering the reader a coherent vision of linear and non-linear optics, and to remove the ambiguities that we have encountered in reference books and articles.

Herzberg-Teller Effect on the Vibrationally Resolved Absorption Spectra of Single-Crystalline Pentacene at Finite Temperatures

The line shape of an electronic spectrum conveys the coupling between electronic and vibrational degrees of freedom. In the present study, the light absorption spectra of single-crystalline pentacene were measured by polarized UV-vis microscopy at 77, 185, and 293 K. The vibronic coupling encoded in each spectrum was resolved by the Herzberg-Teller theory that considers the contributions from the Franck-Condon (FC) factor, Franck-Condo/Herzberg-Teller (FC/HT) interference, and Herzberg-Teller (HT) coupling. Specifically, excitation energies, electronic transition dipole moments, and their nuclear gradients were evaluated by the GW method to ensure numerical accuracy, while the computationally efficient density function theory was employed to determine the optimized structures and vibrational normal modes. For every pair of electronic transition and normal mode that gives rise to a strong vibronic transition intensity, we examined their spatial characteristics by projecting them onto the three crystal axes. It was found that all normal modes strongly coupled to the lowest-lying a-polarized electronic transitions oscillate along axis a, whereas none of their counterparts for the lowest-lying b-polarized electronic transitions is predominantly along axis b. This notable difference on the alignment between the electronic transition and molecular vibration could help the directional control of charge dissociation and/or spin separation. Moreover, a significant variance of the destructive FC/HT interference was discovered with increasing temperatures that can well explain the a-polarized fading tableland near 650 nm. Finally, the importance of HT coupling was corroborated by comparing its intensity with those of FC factor and FC/HT interference. Taken all together, the vibrational dependence of the electronic wave function is critical to resolve the light absorption spectra of single-crystalline pentacene and its temperature effects, facilitating the systematic design of functional optical materials based on pentacene and its derivatives.

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.