Raman Excitation Profiles Research Articles

Pre‐resonance Raman excitation profile of the 3400 cm−1 mode of liquid water

AbstractThe absolute differential Raman scattering cross‐section of the 3400 cm−1 symmetric vibrational mode of liquid water was measured for the excitation wavelength range 450–200 nm. Available data on the Raman cross‐section are critically reviewed. Although the values obtained by most authors at 488 nm excitation are in good agreement, the values are widely different for a few other reported excitation wavelengths. Direct comparison of these results with those measured in this work is not strictly valid owing to the large differences in experimental procedures and methods of data analysis. The excitation wavelength dependence of the Raman cross‐section (excitation profile) was fitted to a single‐term Albrecht A‐term pre‐resonance approximation formula (AATPA). A best fit was obtained for the value of the resonant state, νe = 89 100 cm−1 and the coupling constant, K = 39.2 × 10−27 cm2 (integrated). The data show consistent lower (than predicted) values for wavelengths down to about 220 nm and higher values below this. The imponderable errors in the implementation of corrections at very short wavelengths could well have given rise to a systematic error in this excitation wavelength region. Copyright © 2001 John Wiley & Sons, Ltd.

Time Resolved Resonance Raman, Ab Initio Hartree−Fock, and Density Functional Theoretical Studies on the Transients States of Perfluoro-p-Benzoquinone

In recent times, perfluorinated organic compounds have been investigated extensively to understand the influence of perfluorination on structure−reactivity correlations. Here, we report the time-resolved resonance Raman (TR3), ab initio Hartree−Fock (HF), and density functional theoretical (DFT) studies on the photogenerated transient states of perfluoro-p-benzoquinone (Fluoranil, FA). In particular, for the triplet excited states, radical anion and ketyl radical Raman spectra have been recorded. The observed Raman excitation profiles and the decay rate constants of triplet excited states of FA satisfactorily reproduce, respectively, the absorption spectra and decay rate constants reported previously from transient absorption studies. The structure and vibrational spectra of all these intermediates of FA have been calculated using both ab initio unrestricted Hartree−Fock (UHF) and density functional (UBP86) methods with standard 6-31G(d) basis set. The assignments for all the experimentally observed resonance Raman bands are made using the calculated frequencies and the normal coordinate analysis. Potential energy distributions (PEDs) are also presented. Perfluoro effect is found to be more pronounced in the triplet excited state than in the ground state or the radical anion, whereas the effect in the ground state seems to be higher than that in the radical anion. The lowest triplet excited state of FA has been identified as the ππ* state (3B3G) in nonpolar solvents and the nπ* state (3B1G) in polar solvents. The solvent polarity appears to play a major role in the nature of the lowest triplet excited states, since these two states are very close to each other.

Solvent effects on ground and excited electronic state structures of p-nitroaniline

Resonance Raman intensities of p-nitroaniline, a prototypical “push–pull” chromophore with a large first hyperpolarizability (β), have been measured in dilute solution in five solvents having a wide range of polarities (cyclohexane, 1,4-dioxane, dichloromethane, acetonitrile, and methanol) at excitation wavelengths spanning the strong near-ultraviolet charge-transfer absorption band. The absolute Raman excitation profiles and absorption spectra are simulated using time-dependent wave packet propagation techniques to determine the excited-state geometry changes along the five or six principal Raman-active vibrations as well as estimates of the solvent reorganization energies. The total vibrational reorganization energy decreases and the solvent reorganization energy increases with increasing solvent polarity in all solvents except methanol, where specific hydrogen-bonding interactions may be important. The dimensionless normal coordinate geometry changes obtained from the resonance Raman analysis are converted to actual bond length and bond angle changes with the aid of normal mode coefficients from a ground-state density functional theory calculation. The geometry changes upon electronic excitation involve predominantly the Cphenyl–Nnitro, N–O, and phenyl C2–C3 bond lengths, with little involvement of the amino group. Nonresonant Raman spectra in 1,4-dioxane, dichloromethane, ethyl acetate, acetone, acetonitrile, and methanol show only a very small solvent dependence of the vibrational frequencies. This suggests that changing the solvent affects the excited state more than the ground state, calling into question two-state models that treat the ground and charge-transfer excited states as linear combinations of neutral and zwitterionic basis states with solvent dependent coefficients.

Complex Raman amplitude recovery and dynamics from the Raman excitation profile: application to iodobenzene and azulene

AbstractRecovering the complex amplitude or phase from an intensity, which is given as the modulus square of the amplitude, is a problem common to diverse physical areas. The focus of this paper is on the Raman excitation profile — the Raman intensity of a mode as a function of the excitation energy — which is given by the modulus square of the Raman amplitude. Three methods, with different principles, are presented for amplitude or phase recovery: the dispersion or energy‐frame method; the z‐transform or time‐frame method; and the maximum entropy method. A comparison function technique is introduced to address the usual situation where the measured intensity is band‐limited. There are two classes of problems, minimum phase and non‐minimum phase, and the mapping from the intensity to the complex amplitude may not be unique. The non‐minimum phase situation occurs when the analytically extended complex amplitude, with the energy E replaced by ξ = γ − iE, has zeros in the right‐half complex plane. These zeros have implications for the system dynamics. An algorithm is presented to search for the critical zeros. The characteristics of the zeros are studied with a two‐mode harmonic model, and the theory is applied to the Raman excitation profiles of iodobenzene and azulene. Copyright © 2001 John Wiley & Sons, Ltd.

Study of solvent effects on the molecular structure and the reorganization energies of 4‐nitro‐4′‐dimethylaminoazobenzene using resonance Raman intensities

AbstractResonance Raman spectroscopy was used to probe the effect of solvent polarity on the molecular structure and isomerization dynamics of 4‐nitro‐4′‐dimethylaminoazobenzene (DA). In addition, the influence of the polarity and the relaxation behavior of the solvent on the mode‐specific vibrational and solvent reorganization energies was investigated. Raman spectra were recorded in solvents with different polarity parameters, n‐hexane and benzene, using 11 excitation wavelengths in the range 450–550 nm, scanning every 10 nm using the tunable laser pulse output of an optical parametric oscillator. It was observed that the solvent polarity plays a major role in influencing the excited‐state potential energy surfaces of DA. The plot of resonance Raman intensities versus excitation wavelength (Raman excitation profiles) of the Franck–Condon‐active fundamentals of DA shows that in n‐hexane, the locally excited state is more favorable than that in benzene where the charge‐transfer state is more stable. The distortions along the N N and C–N stretching vibrations of DA in n‐hexane and benzene infer isomerization via rotation in n‐hexane and inversion in benzene. The increase in solvent and vibrational reorganization energies with increasing solvent polarity parameter suggests greater distortion in the Franck–Condon excited state following photoexcitation. Copyright © 2001 John Wiley & Sons, Ltd.

A review on linear and non‐linear resonance Raman spectroscopy of the conjugated system polydiacetylene

AbstractWe review resonance Raman investigations of the linear conjugated π‐electron system of polydiacetylenes embedded in single‐crystalline monomer matrices. Absorption, luminescence, resonance Raman and resonance CARS spectroscopy were used to investigate vibronic properties of the polymers. As an example, we discuss the theoretical model applied for the simulation of the resonance Raman line and excitation profiles obtained from the polymer chains in FBS, TS/FBS and TS6 diacetylene single crystals. The theory is based on a Franck– Condon model, which considers a chain length dependence. The electronic transition energies and matrix elements were calculated by means of an LCAO MO (linear combination of atomic orbitals molecular orbital) calculation in the Hückel approach. An interaction between ensemble and intramolecular properties due to defects can be shown. Besides the well known polymer absorption, the partially polymerized single crystals of the diacetylenes under investigation show blue‐shifted absorption bands in sharply separated crystal areas. The spectroscopic investigations of this chromism revealed that most probably growth defects in {111} crystal growth sectors influence the side‐group geometry of the polymer chains. Using femtosecond time‐resolved CARS spectroscopy, the dynamics of the vibrational excitation in the electronic ground and excited states can be investigated. Here, we report the controlled selective excitation of the polymers. We used a CARS scheme where the difference of pump and Stokes laser wavelengths is chosen to achieve resonance with several chain modes of the polydiacetylenes. By varying the timing and the phases of the femtosecond laser pulses, the timing and the intensity of these modes can be strongly influenced. Copyright © 2001 John Wiley & Sons, Ltd.

The Mechanism of Energy Transfer in the Bacterial Photosynthetic Reaction Center

In the accompanying paper (Scholes, G. D.; Jordanides, X. J.; Fleming, G. R. J. Phys. Chem. 2001, 105, 1640), a generalization of Forster theory is developed to calculate electronic energy transfer (EET) in molecular aggregates. Here we apply the theory to wild-type and mutant photosynthetic reaction centers (RCs) from Rb. sphaeroides, as well as to the wild-type RC from Rps. viridis. Experimental information from the X-ray crystallographic structure, resonance Raman excitation profiles, and hole-burning measurements are integrated with calculated electronic couplings to model the EET dynamics within the RC complex. Optical absorption and circular dichroism spectra are calculated at various temperatures between 10 K and room temperature, and compare well with the experimentally observed spectra. The calculated rise time of the population of the lower exciton state of P, P-, as a result of energy transfer from the accessory bacteriochlorophyll, B, to the special pair, P, in Rb. sphaeroides (Rps. viridis) w...

Subpicosecond solvent dynamics in charge-transfer transitions: challenges and opportunities in resonance Raman spectroscopy.

Resonance Raman spectroscopy is an established tool for the determination of structure and dynamics in electronically excited states. In condensed-phase systems, Raman excitation profiles and electronic absorption spectra depend on changes in molecular geometry and solvation structure induced by electronic excitation. Recent studies of solvent isotope effects on resonance Raman intensities in charge-transfer excitations reveal solvent dynamics taking place on a subpicosecond time scale and vibrational mode-specific solute-solvent interactions. These discoveries present challenges to the current working theories for analysis of resonance Raman and absorption spectra.

Post-resonance Raman and theoretical studies on 1,3,2,4-benzodithiadiazines, formally anti-aromatic compounds

DFT calculations on 1,3,2,4-benzodithiadiazine (1) and its 5-F (2), 6-Cl (3) and 7-CH3 (4) derivatives showed these compounds to have essentially non-planar molecular conformations. The vibrational (IR and Raman) spectra of 1–4 were measured and assigned on the basis of the results of the theoretical calculations. NSN anti-symmetric and symmetric stretching modes were found at ∽1220–1230 and ∽965–975 cm−1, respectively. Resonance Raman spectra of 1–4 were obtained with the 514.5, 496.5, 488.0, 476.5 and 457.9 nm excitation lines of an Argon laser, and a post-resonance Raman effect was observed. The Raman excitation profiles revealed a weak but definite π-interaction of the carbocyclic and heterocyclic parts of the molecules which became stronger on going from 1 to 2. Comments on the possible existence of antiaromaticity for the title compounds in a planar conformation are given.

Raman and absorption spectrum of mass-selected lutetium dimers in argon matrices

We report on the absorption, resonance Raman, and Raman excitation profile spectra of mass selected lutetium dimers in argon matrices. Two broad absorption regions were found between 415 and 545 nm with peaks at 418, 433, 452, 472, 505, 540 nm, and between 635 and 660 nm with peaks at 640 and 652 nm. Resonance Raman spectra were obtained by excitation into one of these regions with the visible light of an Ar ion laser between 458 and 514 nm. Two progressions were assigned to lutetium dimers. We interpret them to represent two distinct electronic states: A ground-state X and an excited state A. For the ground (X) state, we obtain ωe=121.6±0.8 cm−1 with ωexe=0.16±0.10 cm−1, leading to a spectroscopic dissociation energy of 2.9±1.8 eV, and force constant ke=0.76±0.01 mdyne/Å. The lowest excited electronic state (A) has an origin at 210.7 cm−1 and almost identical vibrational parameters. Comparison of spectroscopic properties of lanthanide dimers is discussed, and evidence is presented that lutetium should occupy the position normally given to lanthanum in the periodic table.

Excited state molecular configuration of biphenyl

Contributions of different electronic states to Raman scattering have been studied by critical analyses of Raman excitation profiles of several normal modes of vibration of biphenyl. In this context, the possible structure and other interesting properties of the molecule in the excited electronic states have been discussed. It is found that the first allowed transition to the excited state occurs at 248 nm whose major geometry change involves CC stretching vibrations, whereas, for the second allowed transition (corresponding to 205 nm) inter (i.e., substituent-sensitive) and intra (i.e., ring-breathing) ring CC stretching and in plane CCH angle bending modes are important.

Preparation and characterization of gold and ruthenium colloids in thin zinc oxide films

Gold and Ruthenium colloids have been introduced into thin films of ZnO by spin coating metal ion-doped solutions of zinc acetylacetonate onto quartz or Pt substrates, followed by heating to 400°C. Both types of film exhibit a blue tint due to Mie light absorption of the metal nanoparticles. Analysis of the visible absorption spectra suggests particle radii on the order of 2 nm. However, electron micrographs reveal some larger particles and particle aggregates as well. While the Au colloid absorption band peaks at approximately 575 nm, Raman excitation profiles of the colloidal films exhibit red-shifted maxima near 700 nm. We attribute the striking Raman intensity enhancement at an excitation wavelength of ∼700 nm to Surface-enhanced Raman (SER) scattering of the E 1 mode of ZnO. Ru colloids also induce SER scattering from a residual RuO 2 layer that apparently coats the Ru metal particle. In this case, the SER scattering is greatest when the laser excitation is tuned into the maximum of the Ru colloid absorption band, in accord with the so-called electromagnetic theory of SER scattering. While not all of the Ru in Ru-doped films is prepared in the zero oxidation state, XPS measurements show that all of the gold in the gold-doped films is in the zero oxidation state.

Raman Excitation Profiles with Self-Consistent Excited-State Displacements

Raman excitation profiles (REPs) are computed in the time domain for harmonic modes with excited-state displacement g and measured at multiple excitation frequencies ωL in a bimetallic complex with a metal-to-metal charge-transfer band and 10 coupled modes. Self-consistency is required because the time evolution depends on all displaced modes rather than the individual intensities used in the short-time approximation for g(0). Self-consistency ensures that g is independent of ωL for a single resonance. Exact REPs for displaced oscillators show the importance of vibronic lifetimes and marked deviations from the short-time approximation at molecular frequencies. Spectra at multiple ωL indicate scattering from another electronic excitation with displacement h that is also found self-consistently. Mode-specific Raman data at multiple ωL require additional parameters to yield reliable structural information. Harmonic oscillators with self-consistent displacements are a first approximation to systems with many ...

Resonance Raman Spectroscopy of Met121Glu Azurin

The resonance Raman spectra for Pseudomonas aeruginosa Met121Glu azurin have been measured at wavelengths throughout the 600-nm absorption band at low and high pH. The spectra of the mutant at pH 3.5 and 7.0 are identical. Analysis of the 600-nm absorption band and resulting resonance Raman excitation profiles under both pH conditions indicates that the excited-state distortions in this mutant are smaller than those in the native protein.

Raman Spectrum of Mass-Selected Terbium Dimers in Argon Matrixes

We report on the absorption, resonance Raman, and Raman excitation profile spectra of mass-selected terbium dimers in argon matrixes. An absorption band found between 620 and 680 nm was assigned to terbium dimers. The Raman excitation profile of dimer frequencies resembles closely this absorption spectrum. Unlike other lanthanide dimers, resonance Raman spectra of terbium dimer, obtained by excitation into this region with tunable dye laser radiation, gives two progressions with one up to 7 Stokes transitions, and the other up to 4 transitions. We interpret these to represent two distinct electronic states X and A. For the ground (X) state, we obtain ωe = 137.6 ± 0.4 cm-1 with ωexe = 0.31 ± 0.05 cm-1, leading to a spectroscopic dissociation energy of 1.9 ± 0.3 eV, force constant ke = 0.88 ± 0.01 mdyn/Å. The first electronic (A) state has an origin at 313.9 cm-1 with a progression in ωe = 136.3 ± 0.2 cm-1 and ωexe = 0.14 ± 0.05 cm-1, leading to an excited-state spectroscopic dissociation energy of 4.1 ± 1.2 eV.

Raman Excitation Profiles and Excited State Molecular Configurations of Three Isomeric Acetylpyridines and Acetophenone

Contributions of different electronic states to Raman scattering were studied by critical analyses of Raman excitation profiles (REPs) of several normal modes of vibration of three isomeric acetylpyridines and acetophenone. In this context, the possible structures and other interesting properties of the four molecules in the excited electronic states were also discussed. Most likely a singlet state, lying in the vacuum ultraviolet region with respect to the ground state, is found to play a very significant role in the scattering phenomena.

Interference Effects of Multiple Excited States in the Resonance Raman Spectroscopy of CpCoCOD

The resonance Raman excitation profiles of cyclopentadienylcobalt cyclooctadiene, CpCoCOD, contain pronounced decreases in intensity in the region of the lowest energy ligand field absorption band. This deenhancement is caused by interference between the ligand field state and nearby metal to ligand charge-transfer states. The changes in the Raman intensities are quantitatively calculated. The relevant electronic excited states are assigned by using single-crystal absorption spectroscopy, and the vibrational modes are assigned by deuteration studies. Distortions along the metal−ligand vibrational normal coordinates in the interfering states are calculated. The origin of the deenhancement, the relative signs of the bond length changes, and the relationship of the bond length changes to the molecular orbitals involved in the transitions are discussed.

Spectroscopy of mass-selected gadolinium dimers in argon matrices

We report on the absorption, resonance Raman, and Raman excitation profile spectra of mass-selected gadolinium dimers in argon matrices. Two dimer absorption bands were found between 570 and 660 nm. The Raman excitation profile resembles the dimer absorption spectrum very well. Resonance Raman spectra, obtained by excitation into this region with dye laser radiation, give a single progression with up to four Stokes transitions. These data give ωe=138.7±0.4 cm−1 with ωexe=0.3±0.1 cm−1, leading to a spectroscopic dissociation energy of 2.1±0.7 eV.

Intermolecular hydrogen abstraction from triplet excited state of decafluorobenzophenone: a Raman investigation

The influence of the structure of the triplet excited state of benzophenone (bp) and decafluorobenzophenone (dfbp) on the reactivity towards hydrogen abstraction was studied by time-resolved resonance Raman spectroscopy. The resonance Raman spectra and Raman excitation profiles were recorded for both the triplet excited state and the ketyl radical. Vibrational spectral assignments were made for all the intermediate states. Based on the analysis of the spectra, it was concluded that the enhanced reactivity towards hydrogen atom abstraction of dfbp over bp is due to (a) the non-planar structure of the triplet excited state of dfbp, thus increasing the accessibility for the CO group, and (b) the highly polarized and electrophilic nature of the CO group in the triplet excited state of dfbp. Copyright © 2000 John Wiley & Sons, Ltd.

Raman excitation profiles and excited state molecular configurations of three isomeric phenyl pyridines

Contributions of different electronic states to Raman scattering have been studied by critical analyses of Raman excitation profiles (REPs) of several normal modes of vibration of three isomeric phenyl pyridines. In this context, possible structures and other interesting properties of the three molecules in the excited electronic states have been discussed. Normal mode characteristics are also described. Most likely a singlet state, lying in the vacuum ultraviolet region with respect to the ground state, is found to be playing a very significant role in the scattering phenomena.