Resonance hyper-Raman Spectra Research Articles

Surface-enhanced hyper-Raman scattering of Rhodamine 6G isotopologues: Assignment of lower vibrational frequencies.

We report a comprehensive experimental and theoretical study of the lower-wavenumber vibrational modes in the surface-enhanced hyper-Raman scattering (SEHRS) of Rhodamine 6G (R6G) and its isotopologue R6G-d4. Measurements acquired on-resonance with two different electronic states, S1 and S2, are compared to the time-dependent density functional theory computations of the resonance hyper-Raman spectra and electrodynamics-quantum mechanical computations of the SEHRS spectra on-resonance with S1 and S2. After accounting for surface orientation, we find excellent agreement between experiment and theory for both R6G and its isotopologue. We then present a detailed analysis of the complex vibronic coupling effects in R6G and the importance of surface orientation for characterizing the system. This combination of theory and experiment allows, for the first time, an unambiguous assignment of lower-wavenumber vibrational modes of R6G and its isotopologue R6G-d4.

Assessment of mode-mixing and Herzberg-Teller effects on two-photon absorption and resonance hyper-Raman spectra from a time-dependent approach

A time-dependent approach is presented to simulate the two-photon absorption (TPA) and resonance hyper-Raman scattering (RHRS) spectra including Duschinsky rotation (mode-mixing) and Herzberg-Teller (HT) vibronic coupling effects. The computational obstacles for the excited-state geometries, vibrational frequencies, and nuclear derivatives of transition dipole moments, which enter the expressions of TPA and RHRS cross sections, are further overcome by the recently developed analytical excited-state energy derivative approaches in the framework of time-dependent density functional theory. The excited-state potential curvatures are evaluated at different levels of approximation to inspect the effects of frequency differences, mode-mixing and HT on TPA and RHRS spectra. Two types of molecules, one with high symmetry (formaldehyde, p-difluorobenzene, and benzotrifluoride) and the other with non-centrosymmetry (cis-hydroxybenzylidene-2,3-dimethylimidazolinone in the deprotonated anion state (HDBI(-))), are used as test systems. The calculated results reveal that it is crucial to adopt the exact excited-state potential curvatures in the calculations of TPA and RHRS spectra even for the high-symmetric molecules, and that the vertical gradient approximation leads to a large deviation. Furthermore, it is found that the HT contribution is evident in the TPA and RHRS spectra of HDBI(-) although its one- and two-photon transitions are strongly allowed, and its effect results in an obvious blueshift of the TPA maximum with respect to the one-photon absorption maximum. With the HT and solvent effects getting involved, the simulated blueshift of 1291 cm(-1) agrees well with the experimental measurement.

Investigation of tautomeric composition of cytosine by multiphoton excitation spectra. I. Resonance hyper-Raman spectra

For the first time the quantum-mechanical calculations of intensity distribution in the resonance hyper-Raman spectra of the six tautomeric forms of cytosine were performed. It has been confirmed that in the aqueous solution of cytosine (pH 3.0) there exist several molecular structures: cytosine in the canonical amino-oxo tautomeric form, cation, as well as cys-imino-oxo and cys-amino-hydroxy tautomers; in the gaseous phase at 235 degrees C cys-imino-oxotautomer dominates with the presence of small amount of cys-, trans-, amino-hydroxy tautomers and the caconical form of cytosine.

Surface-Enhanced Hyper-Raman Scattering Elucidates the Two-Photon Absorption Spectrum of Rhodamine 6G

The surface-enhanced hyper-Raman (SEHRS) spectra of Rhodamine 6G (R6G) are measured for two different excitation energies: ∼25 000 cm–1 (∼810 nm) and ∼19 000 cm–1 (1030 nm). The collected spectra are compared to time-dependent density functional theory simulations of the resonance hyper-Raman spectra for the same excitation energies. The analysis of molecular orbital changes in these nonlinear transitions elucidates the mechanism of vibronic enhancement. This analysis is used to elucidate features in the two-photon absorption spectra of R6G.

Solvent effects on resonance Raman and hyper-Raman scatterings for a centrosymmetric distyrylbenzene and relationship to two-photon absorption

Resonance Raman (RR) and resonance hyper-Raman (RHR) spectra have been obtained for a nominally centrosymmetric dialkylammonium-substituted distyrylbenzene that was previously shown to exhibit a strong solvent polarity effect on its two-photon absorption strength. Spectra have been obtained in acetonitrile, methanol, dimethylsulfoxide, and water. In each solvent, the RR and RHR spectra show very similar intensity patterns in the strongest lines in both the C=C and C-C stretching regions, but the RHR peaks are shifted to higher energies by 5-10 cm(-1). These are interpreted as vibrations of similar mode character but different symmetries, with u symmetry modes observed in RHR and g symmetry modes in RR. In all solvents the nonzero chromophore contribution to the hyper-Rayleigh scattering indicates that the center of symmetry is broken to some degree, and this effect is more pronounced in water than in the other three solvents. The ratio of RR intensity to one-photon fluorescence intensity shows an approximate inverse relationship with the fluorescence quantum yield, consistent with the approximate solvent independence of the one-photon transition strength implied by the linear absorption spectra. The ratio of RHR intensity to two-photon fluorescence intensity is smaller than the corresponding RR to one-photon fluorescence ratio in all four solvents.

Resonance Hyper-Raman Excitation Profiles and Two-Photon States of a Donor−Acceptor Substituted Polyene

Resonance Raman and resonance hyper-Raman spectra and excitation profiles have been measured for a "push-pull" donor-acceptor substituted conjugated polyene bearing a julolidine donor group and a nitrophenyl acceptor group, in acetone at excitation wavelengths from 485 to 356 nm (two-photon wavelengths for the nonlinear spectra). These wavelengths span the strong visible to near-UV linear absorption spectrum, which appears to involve at least three different electronic transitions. The relative intensities of different vibrational bands vary considerably across the excitation spectrum, with the hyper-Raman spectra showing greater variation than the linear Raman. A previously derived theory of resonance hyper-Raman intensities is modified to include contributions from purely vibrational levels of the ground electronic state as intermediate states in the two-photon absorption process. These contributions are found to have only a slight effect on the hyper-Rayleigh intensities and profiles, but they significantly influence some of the hyper-Raman profiles. The absorption spectrum and the Raman, hyper-Rayleigh, and hyper-Raman excitation profiles are quantitatively simulated under the assumption that three excited electronic states contribute to the one- and two-photon absorption in this region. The transition centered near 400 nm is largely localized on the nitrophenyl group, while the transitions near 475 and 355 nm are more delocalized.

Resonance hyper-Raman excitation profiles of a donor-acceptor substituted distyrylbenzene: One-photon and two-photon states

Resonance Raman and resonance hyper-Raman spectra of the "push-pull" conjugated molecule 1-(4'-dihexylaminostyryl)-4-(4"-nitrostyryl)benzene in acetone have been measured at excitation wavelengths from 485 to 356 nm (two-photon wavelengths for the nonlinear spectra), resonant with the first two bands in the linear absorption spectrum. The theory of resonance hyper-Raman scattering intensities is developed and simplified using assumptions appropriate for intramolecular charge-transfer transitions of large molecules in solution. The absorption spectrum and the Raman, hyper-Rayleigh, and hyper-Raman excitation profiles, all in absolute intensity units, are quantitatively simulated to probe the structures and the one- and two-photon transition strengths of the two lowest-energy allowed electronic transitions. All four spectroscopic observables are reasonably well reproduced with a single set of excited-state parameters. The two lowest-energy, one-photon allowed electronic transitions have fairly comparable one-photon and two-photon transition strengths, but the higher-energy transition is largely localized on the nitrophenyl group while the lower-energy transition is more delocalized.

Resonance Raman and hyper‐Raman spectroscopy of organic chromophores for second‐order nonlinear optics

AbstractTwo‐photon resonant hyper‐Raman spectroscopy from large molecules in solution is discussed as a method for probing excited electronic states that are both one‐ and two‐photon allowed. A number of electron donor–acceptor substituted conjugated organic molecules that have large first hyperpolarizabilities (β) have recently been shown to give strong resonance hyper‐Raman (RHR) spectra in solution. The quantum mechanical formulation of RHR scattering is reviewed, and practical approaches to calculating RHR spectra and excitation profiles are proposed by reference to methods already developed for simulating linear resonance Raman spectra and profiles. The potential benefits of a close interplay between experiments, empirical simulations, and electronic structure calculations of ground‐state and excited‐state properties are emphasized. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Tunable resonance hyper-Raman spectroscopy of second-order nonlinear optical chromophores.

Two-photon-resonant hyper-Raman spectra are reported for three "push-pull" conjugated organic chromophores bearing -NO(2) acceptor groups, two dipolar and one octupolar. The excitation source is an unamplified picosecond mode-locked Ti:sapphire laser tunable from 720 to 950 nm. The linear resonance Raman spectra of the same molecules are measured using excitation from the laser second harmonic. Excitation on resonance with the lowest-lying band in the linear absorption spectrum yields nearly identical resonance Raman and resonance hyper-Raman spectra. However, excitation into a region that appears to contain more than one electronic transition gives rise to different intensity patterns in the linear and nonlinear spectra, indicating that different transitions contribute differently to the one-photon and two-photon oscillator strength. The promise of the hyper-Raman technique for examining electronic transitions that are both one- and two-photon allowed is discussed.

Resonance hyper-Raman scattering from conjugated organic donor-acceptor "push-pull" chromophores with large first hyperpolarizabilities.

Resonance hyper-Raman spectra have been obtained using 1064 nm excitation for several electron donor-acceptor-substituted, pi-conjugated "push-pull" molecules that have large second harmonic hyperpolarizabilities. The hyper-Raman spectra are nearly identical to the resonance Raman spectra measured with 532 nm excitation. This indicates that both the second harmonic hyperpolarizability and the linear absorption are dominated by the same, single electronic transition that is both one- and two-photon allowed. Comparison of resonance Raman and resonance hyper-Raman spectra is proposed as an experimental test of the common two-electronic-state model for the first hyperpolarizability.

Intracavity resonance hyper-Raman scattering with a synchronously pumped mode-locked dye laser

The first resonance hyper-Raman spectra of liquids are made possible using an intracavity scattering geometry in a synchronously pumped picosecond stilbene 420 dye laser. Comparisons are made with the corresponding UV resonance Raman spectra, which are generated by the second harmonic of the cavity dumped output. Information about both the vibrational and electronic states of sample molecules is obtained.