Electronic Sum Frequency Generation Spectroscopy Research Articles

Theoretical study of electronic sum frequency generation spectroscopy to assess the buried interfaces.

This article provides a comprehensive theoretical background of electronic sum frequency generation (ESFG), a second-order nonlinear spectroscopy technique. ESFG is utilized to investigate both exposed and buried interfaces, which are challenging to study using conventional spectroscopic methods. By overlapping two incident beams at the interface, ESFG generates a beam at the sum of their frequencies, allowing for the extraction of valuable interfacial molecular information such as molecular orientation and density of states present at interfaces. The unique surface selectivity of ESFG arises from the absence of inversion symmetry at the interfaces. However, detecting weak signals from interfaces requires the ultrafast lasers to generate a sufficiently strong signal. By understanding the theoretical foundations of ESFG presented in this article, readers can gain a solid grasp of the basics of ESFG spectroscopy.

Revisiting the π → π* transition of the nitrite ion at the air/water interface: A combined experimental and theoretical study

Broadband deep ultraviolet electronic sum frequency generation spectroscopy allows measurement of electronic |χ(2)|2-spectra at aqueous interfaces, providing considerable improvement over deep ultraviolet electronic second harmonic generation spectroscopy that yields error-prone, pointwise spectra. Reexamination of the π → π* transition of nitrite at the air/water interface reveals that the interfacial |χ(2)|2-spectrum is strikingly similar to the bulk absorption spectrum. Molecular dynamics simulations and SOS-CIS(D0) electronic structure calculations provide no evidence for the previously reported contact ion pair-induced red-shift of the π → π* transition at the air/water interface. Our results thus revise the previous description for nitrite interfacial adsorption as a contact ion pair.

Charge-Transfer-to-Solvent Spectrum of Thiocyanate at the Air/Water Interface Measured by Broadband Deep Ultraviolet Electronic Sum Frequency Generation Spectroscopy.

Measurement of interfacial electronic spectra is a powerful tool for characterizing the properties of ions in physical, biological, environmental, and industrial systems. Here, we describe measurement of the complete charge-transfer-to-solvent (CTTS) spectrum of thiocyanate at the air/water interface using our recently developed femtosecond broadband deep ultraviolet electronic sum frequency generation technique. We find that the lower energy CTTS band characterized in bulk thiocyanate spectra is not observed in the | χ(2)|2-power spectrum of the air/water interface, likely reflecting the different solvation environments, altering of the charge distribution of the ion, and possible ion-ion effects, and that sodium and potassium salts yield identical spectra. Additional experiments and comparison with theoretical calculations are necessary to extract the interesting chemical details responsible for these salient spectral differences.

Using Heterodyne-Detected Electronic Sum Frequency Generation To Probe the Electronic Structure of Buried Interfaces

Organic semiconductors (OSCs) are attractive optoelectronic materials due to their high extinction coefficients, processing advantages, and ability to display unique phenomena such as singlet exciton fission. However, employing OSCs as active electronic components remains challenging, as this necessitates forming junctions between OSCs and other materials. Such junctions can distort the OSC’s electronic properties, complicating the transfer of energy and charge across them. To investigate these junctions, our group has employed an interface-selective technique, electronic sum frequency generation spectroscopy (ESFG), yet one complication in applying ESFG to thin OSC films is they necessarily have two interfaces that can each produce signals. In a conventional ESFG measurement, information regarding the phase of the ESFG signal is lost. However, this information can be recovered with heterodyne detection (HD) techniques. Here, we present experiments and model calculations that illustrate some key advantages offered by HD-ESFG over conventional ESFG measurements for the study of OSC films. Specifically, we report HD-ESFG spectra of N,N′-dimethyl-3,4,9,10-perylenedicarboximide (C1-PDI) thin films that have been grown on SiO2. To implement these measurements, we have constructed an HD-ESFG spectrometer that uses common path optics to maintain a high degree of phase stability over multiple hours. We find that not only does HD-ESFG offer increased sensitivity to weak features in ESFG spectra, but the phase information included in these measurements aids in selectively isolating signals that arise from a specific film interface. Interestingly, we find that resonances in HD-ESFG spectra of C1-PDI are significantly shifted from those in linear absorption spectra of bulk C1-PDI films, suggesting that the intermolecular packing of molecules at film interfaces differs from the bulk.

Electronic sum-frequency generation (ESFG) spectroscopy: theoretical formulation of resonances with symmetry-allowed and symmetry-forbidden electronic excited states

ABSTRACTSum-frequency generation spectroscopy is a powerful tool for analysing molecular species and orientation configurations on a surface or an interface. In addition to conventional vibrational resonant techniques, the electronic resonant (electronic sum-frequency generation [ESFG]) ones have been developed recently. We have established the theoretical formulation of ESFG spectroscopy based on the susceptibility approach, considering resonances with both symmetry-allowed and symmetry-forbidden electronic excited states; derivatives of transition dipole moments and vibronic couplings beyond the Condon approximation were included. With the aid of density functional theory computations, simulated ESFG spectra in resonances with both types of excited states of acetone are demonstrated.

Probing Electronic Structures of Organic Semiconductors at Buried Interfaces by Electronic Sum Frequency Generation Spectroscopy

We use Electronic Sum Frequency Generation Spectroscopy (ESFG) to study the electronic structures at a buried solid/solid interface for the first time. The system is an organic thin film, poly(3-hexylthiophene-2,5-diyl) (P3HT), supported on a silicon surface. The ESFG measurement is only in resonance with electronic (or vibronic) excitations, thus capable of yielding rich information on the band gap and electronic structures of the P3HT film at interfaces. We find the bandgap of P3HT in contact with silicon is 2.2 eV, with a narrowed bandwidth and Lorentzian line shape. This is significantly distinct from the UV–vis spectra of bulk P3HT, which contains multiple broad Gaussian peaks. Our measurement demonstrates at interfaces regioregular P3HT has a uniform electronic structure, which could improve the short circuit currents. The unique capability of ESFG to probe electronic structures at buried interface under atmosphere will be useful for investigating many buried interfaces.

“Up” versus “down” alignment and hydration structures of solutes at the air/water interface revealed by heterodyne-detected electronic sum frequency generation with classical molecular dynamics simulation

We unambiguously demonstrate the "up" versus "down" alignment of a pair of prototypical solute molecules adsorbed at the air/water interface for the first time using heterodyne-detected electronic sum frequency generation spectroscopy. This molecular alignment is also reproduced by classical molecular dynamics (MD) simulation theoretically. Furthermore, the MD simulation indicates distinctly different interface-specific hydration structures around the two solute molecules, which dictate the molecular alignment at the interface. It is concluded that the hydrophilicity difference between the terminal functional groups of the solute governs the molecular orientation and surrounding hydration structures at the interface.

Electric quadrupole contribution to the nonresonant background of sum frequency generation at air/liquid interfaces

We study an electric quadrupole contribution to sum frequency generation (SFG) at air∕liquid interfaces in an electronically and vibrationally nonresonant condition. Heterodyne-detected electronic sum frequency generation spectroscopy of air∕liquid interfaces reveals that nonresonant χ((2)) (second-order nonlinear susceptibility) has a negative sign and nearly the same value for all eight liquids studied. This result is rationalized on the basis of the theoretical expressions of χ((2)) with an electric quadrupole contribution taken into account. It is concluded that the nonresonant background of SFG is predominantly due to interfacial nonlinear polarization having a quadrupole contribution. Although this nonlinear polarization is localized at the interface, it depends on quadrupolar χ((2)) in the bulk as well as that at the interface. It means that the sign of nonresonant χ((2)) bears no relation to the "up" versus "down" alignment of interfacial molecules, because nonresonant χ((2)) has a quadrupolar origin.

“Half-hydration” at the air/water interface revealed by heterodyne-detected electronic sum frequency generation spectroscopy, polarization second harmonic generation, and molecular dynamics simulation

A solute-solvent interaction at the air/water interface was investigated both experimentally and theoretically, by studying a prototypical surface-active polarity indicator molecule, coumarin 110 (C110), adsorbed at the air/water interface with heterodyne-detected electronic sum frequency generation (HD-ESFG) spectroscopy, polarization second harmonic generation (SHG), and a molecular dynamics (MD) simulation. The second-order nonlinear optical susceptibility (chi((2))) tensor elements of C110 at the air/water interface were determined experimentally by HD-ESFG and polarization SHG, and information on "intermediate" polarity sensed by C110 at the interface was obtained by HD-ESFG. An MD simulation and a time-dependent density functional theory calculation were used to theoretically evaluate the chi((2)) tensor elements, which were in good agreement with the experimental results of HD-ESFG and polarization SHG. The microscopic "half-hydration" structure around C110 at the water surface was visualized on the basis of the MD simulation data, with which we can intuitively understand the microscopic origin of the surface activity of C110 and the intermediate polarity sensed by C110 at the air/water interface.

New Insight into the Surface Denaturation of Proteins: Electronic Sum Frequency Generation Study of Cytochrome c at Water Interfaces

In situ characterization of surface denaturation of a protein was realized by newly developed interface-selective multiplex electronic sum frequency generation spectroscopy. The observed electronic spectra of cytochrome c at the air/water interface exhibited a broad feature, which demonstrated coexistence of the nativelike and denatured protein at the interface. This situation of the mixed conformation at the air/water interface did not change in the acidic condition of pH=2 where the protein was completely denatured in the bulk water. In sharp contrast, only native spectrum was observed at the silica/water interface.