Doubly-resonant Sum-frequency Generation Research Articles

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.

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.

Aggregated States of Chalcogenorhodamine Dyes on Nanocrystalline Titania Revealed by Doubly Resonant Sum Frequency Spectroscopy

Broadband doubly resonant sum frequency generation (DR-SFG) spectroscopy was used to probe selectively the vibrational signatures of a series of chalcogenorhodamine dyes tethered to TiO2 either as monomers or as H-aggregates. This selectivity was achieved by virtue of the vibronic coupling associated with the unique electronic transitions for the corresponding states. It was found that clear spectral differences and polarization dependences were observed contingent on whether the outgoing SFG wavelength was in resonance with either the monomer or H-aggregate electronic excited states. Our results also indicate that the orientation and proximity of the xanthylium core to the semiconducting surface play an important role in this vibrational–electronic coupling and may affect the character of the charge-transfer complex. Our DR-SFG results also provide important clues into the anchoring motifs for carboxylic and phosphonic acid groups.

Substrate influence on the interlayer electron-phonon couplings in fullerene films probed with doubly-resonant SFG spectroscopy.

We present doubly-resonant sum frequency generation (DR-SFG) spectra of fullerene thin films on metallic and dielectric substrates as a way to investigate the interplay between nuclear and electronic coupling at buried interfaces. Modal and substrate selectivity in the electronic enhancement of the C60 vibrational signatures is demonstrated for excitation wavelengths spanning the visible range. While the SFG response of the totally symmetric Ag(2) mode of fullerene is distinctly coupled to the optically allowed electronic transition corresponding to the HOMO-LUMO+1 of C60 (ca. 2.6 eV), the T1u(4) vibrational mode appears to be coupled to a symmetry-forbidden HOMO-LUMO transition at lower energies (ca. 2.0 eV). For dielectric substrates, the DR-SFG intensity of the T1u(4) mode shows lack of enhancement for upconversion wavelengths off-resonance with the optically-dark LUMO. However, the T1u(4) mode shows a unique coupling to an intermediate state (∼2.4 eV) only for the fullerene films on the gold substrate. We attribute this coupling to unique interactions at the buried C60/gold interface. These results demonstrate the occurrence of clear electron-phonon couplings at the C60/substrate interfaces and shed light on the impact of these couplings on the optical response of electronically excited fullerene. This coupling may influence charge and energy transport in organic electronic devices mediated by vibrational motions. We also demonstrate a potential use of this added selectivity in chemical imaging.

Non-Condon Effects on the Doubly Resonant Sum Frequency Generation of Rhodamine 6G.

We report first-principles simulations of the doubly resonance sum-frequency generation (DR-SFG) spectrum for rhodamine 6G (R6G). The simulations are done using a time-dependent formalism that includes both Franck-Condon (FC) and Herzberg-Teller (HT) terms in combination with time-dependent density functional theory (TDDFT) calculations. The simulated spectrum matches experiments, allowing a detailed assignment of the DR-SFG spectrum. Our work also shows that non-Condon effects are important and the DR-SFG spectrum of R6G is highly dependent on both FC and HT modes. This is surprising as R6G is known to be a strong FC resonant Raman scatterer. The simulations predict an orientation where the xanthene plane of R6G is perpendicular to the surface with binding through one of the ethyl amine groups. Our results show the importance of first-principles simulations for providing a detailed assignment of DR-SFG experiments, especially for large molecules where such an assignment is complicated due close-lying vibrational modes.

Self-assembled monolayers of cyanoterphenyl terminated alkyl disulfides studied by vibrationally and electronically doubly-resonant sum-frequency generation spectroscopy

Self-assembled monolayers (SAMs) of asymmetric (LC-monomer) and symmetric (LC-dimer) cyanoterphenyl terminated alkyl disulfides were investigated by doubly-resonant sum-frequency generation (DR-SFG) with the help of DFT calculation. In addition to the cyano stretching band, intense bands of the terphenyl were observed due to electronic resonance. Similar coverages on both SAMs were obtained by RAIRS and XPS; i.e., the number density of the terphenyl on LC-monomer SAM was about half of that on the dimer, though the DR-SFG signal of the monomer exceeded that of the dimer. These findings are explained by the difference in the orientational distributions of the terphenyls.

Doubly Resonant Sum Frequency Generation Spectroscopy of Adsorbates at an Electrochemical Interface

In this paper we report the first spectroelectrochemical in situ doubly resonant sum frequency generation (DR-SFG) spectroscopy experiment carried out with a free-electron laser tunable IR source and IR and VIS Optical Parametric Oscillators. We studied the potential-dependent adsorption and reaction of the quinoline dye 4-{2-[1-(2-cyanoethyl)-1,2,3,4-tetrahydroquinolin-6-yl]diazenyl} benzonitrile (CTDB) onto a Cu(100) electrode. SFG band assignments were based on density functional theory calculations. Strong SFG enhancement was obtained in the IR range 1100−1300 cm−1, corresponding to Ar—C≡N stretching, with the VIS beam set at 441.6 nm and in the IR range 2000−2300 cm−1, corresponding to nitrile stretching, with the VIS beam set at 532 nm and the pH adjusted in order to match the bathocromic shift of the adsorbed CTDB chromophore with the input VIS laser frequency. Doubly resonant conditions were achieved via resonance of the VIS input with the electronic structure of the adsorbate orbitals and of the ...

Ab initio calculations of doubly resonant sum-frequency generation second-order polarizabilities of LiH

In the present Letter doubly resonant sum-frequency generation (DR-SFG) second-order polarizabilities ( β) are reported for the lithium hydride molecule. The calculations, which refer to the diagonal longitudinal component of β and ignore rotation, are based on data evaluated using the multireference configuration interaction method with singles and doubles. The DR-SFG results are partitioned into their infrared–visible and visible–infrared components. They are analyzed as a function of the infrared and visible frequencies as well as the number of vibrational states considered in the sum-over-states expressions.

Electron–phonon couplings at C 60 interfaces: a case study by two-color, infrared–visible sum–frequency generation spectroscopy

We demonstrate the ability of doubly resonant sum–frequency generation (DR-SFG) to investigate electron–phonon couplings at C 60–metal interfaces. Due to its coupling to electronic transitions, the totally symmetric A g(2) vibration of C 60 exhibits a huge enhancement of its nonlinear response for sum–frequency energies above the molecular electronic gap. We attribute this resonance to the coupling of the pentagonal pinch mode with the t 1u lowest unoccupied molecular orbital (LUMO) of C 60.

Development of a two-color picosecond optical parametric oscillator, pumped by a Nd:YAG laser mode locked using a nonlinear mirror, for doubly-resonant sum frequency generation spectroscopy

We set up a doubly-resonant sum frequency generation (DR-SFG) spectrometer based on the use of an all-solid-state flash-lamp-pumped Nd:YAG laser that synchronously pumps two parametric oscillators. Pulses as short as 12 ps FWHM are generated by mode locking a Nd:YAG oscillator using a frequency doubling nonlinear mirror combined with a two-photon absorber. The available pump power is shared between a LiNbO 3/AgGaS 2 optical parametric oscillator (OPO), tunable from 3800 to 1100 cm −1 and a BBO OPO tunable from 410 to 2600 nm. Spectral resolution and pulse are 2 and 3 cm −1 in the infrared and visible spectral ranges, respectively. First DR-SFG spectra of self-assembled monolayers on Au are presented.