Raman Scattering Intensities of the NO2 Symmetric Stretching Vibration Band of Some Aromatic Nitrocompounds (II)

The Raman scattering intensities of the NO₂ symmetric stretching vibration band were investigated experimentally for some para and meta substituted nitrobenzenes. The excitation profiles obtained with the six excitation wavelengths from 457.9 to 514.5 nm were presented. The results were examined in terms of the Albrecht-Hutley's theory, and the electronic states involved in the preresonance Raman intensity enhancement were suggested.     

The roles of vibronic coupling and the Duschinsky effect in resonance Raman scattering

In most vibronic treatments of resonance Raman scattering it is assumed that the normal coordinates of electronically excited states are not rotated relative to those of the ground state, i.e., the Duschinsky effect is neglected. In the present paper this assumption is critically examined for the case of resonance Raman scattering by a non-totally symmetric vibrational mode. For consistency also the second-order Herzberg-Teller coupling is considered. It is shown that these second-order couplings introduce asymmetry in the excitation profile of appearance similar to that due to nonadiabatic coupling. Also it is shown that even small rotations give rise to noticeable effects in the polarization dispersion curve. Analytical formulas for the Franck-Condon factors and for the Raman tensor (valid for an arbitrary number of normal modes) for the case of small rotations are given, and illustrative calculations of excitation and polarization dispersion curves for a two mode system are presented and discussed.     

Radiative lifetimes and absolute oscillator strengths for the SiOAΠ-XΣ transition

Measurement of a radiative lifetime of 9.6 ± 1.0 nsec for the SiOA¹Π-state is reported. RKR Franck-Condon factors have been computed and, with these, absolute oscillator strengths are calculated. A band system at ∼ 3022Å has been found that may be due to SiO⁺, but the band is not resolved at our experimental resolution, and identification remains uncertain. The upper state of the carrier of this spectrum has a radiative lifetime of 8.3 ± 0.8 nsec.     

Electronic and Resonance Raman Spectra of a Multibridged Iron Porphyrin

Abstract

Reported are the electronic and resonance Raman (rR) spectra of a multibridged iron porphyrin derived from meso-tetra(4-pyridil)porphynate iron(II) by complexing the pyridine residues with [Ru^II(edta)]^2- ions. The polymetallated system exhibits the characteristic Soret band at 428 nm (log ε = 5.2) and a shoulder around 465 nm (4.6). The rR enhancement of the porphyrin vibrational modes is similar to those previously reported for typical porphyrins; however, an additional feature appears around 465 nm, associated with the selective enhancement of some pyridine vibrational modes. Based on the rR excitation profiles the absorption band at 465 nm was assigned to a Ru^II-to-pyridine dπ→Pπ? charge-transfer transition.     

Raman study of Fano interference in p‐type doped silicon

As the silicon industry continues to push the limits of device dimensions, tools such as Raman spectroscopy are ideal to analyze and characterize the doped silicon channels. The effect of inter‐valence band transitions on the zone center optical phonon in heavily p‐type doped silicon is studied by Raman spectroscopy for a wide range of excitation wavelengths extending from the red (632.8 nm) into the ultra‐violet (325 nm). The asymmetry in the one‐phonon Raman lineshape is attributed to a Fano interference involving the overlap of a continuum of electronic excitations with a discrete phonon state. We identify a transition above and below the one‐dimensional critical point (E = 3.4 eV) in the electronic excitation spectrum of silicon. The relationship between the anisotropic silicon band structure and the penetration depth is discussed in the context of possible device applications. Copyright © 2010 John Wiley & Sons, Ltd.     

Excitation profiles of resonant coherent Raman scattering by impurity molecules

The amplitudes and the excitation profiles of coherent resonant Raman scattering (coherent anti‐Stokes and Stokes Raman scattering (CARS and CSRS)) by impurity centers in crystals and/or molecules in solutions are studied, taking into account the coherence of the excited mode and the inhomogeneity of the environment. The CARS and CSRS excitation profiles can directly be related to one‐photon absorption when using the transform theory known from spontaneous resonant Raman scattering. The enlargement of the amplitude of the active mode reveals itself in an enhancement of the vibrational overtones. At large amplitudes, a splitting in the profiles caused by the contribution of two turning points of a strong coherent vibration is observed. The inhomogeneous broadening is different in the excitation profiles of CARS and CSRS. The overlapping contributions to spectrally narrow and broad transitions lead to Fano peculiarities. The effects are illustrated by model calculations. Copyright © 2011 John Wiley & Sons, Ltd.     

Model studies on the systematics and interpolation of Franck-Condon factor arrays

A simple harmonic oscillator approximation is used to derive approximate algebraic expressions for low vibrational quantum number Franck-Condon factors for band systems for which molecular data are sparse. These simple expressions involve a transition parameter u which is characteristic of the electronic transition involved. The expressions provide Franck-Condon factors which agree quite well with RKR values when these are available. The formulae are used to explain quantitatively the dependence on u of the systematic properties of Franck-Condon factor arrays and to provide a method for their interpolation. The formulae are also used to describe quantitatively the dependence on u of the gross appearance of molecular band systems in emission and absorption.     

Sättigungseffekte und Linienaufspaltung bei der stimulierten Resonanz-Raman-Streuung

Saturation Effects and Line Splitting of the Stimulated Resonance Raman Scattering The gain of the stimulated resonance Raman scattering (SRRS) is calculated for the case of any sharp resonance. It is shown that under steady-state conditions for longitudinal excitation in direction of the cell the saturation of the absorption is a necessary condition to find a sufficient large gain. Therefore, in the case of a Franck-Condon excitation the SRRS with an excited initial level is only possible, whereas for a O—O-excitation the SRRS with the ground or an excited state as initial state is equally probable. Changing of the population by stimulated fluorescence does not influence this assertion.     

Orientation studies by N.M.R. spectroscopy using a lyotropic liquid crystal solvent

Excitation profile of resonance enhanced Raman lines, due to totally symmetric motions of soluble cis-polyacetylene, show a resolved Frank-Condon structure which reproduces the one in the electronic absorption spectrum. A model calculation which uses two excited electronic states coupled by intra-band vibronic interaction, three vibrational modes, homogeneous and inhomogeneous broadening and a small contribution from pre-resonance states, has been worked out to fit the experimental excitation profiles in the framework of the band model approach like that used for metals in solid state language.     

Lifetime of pair excitations in nuclear fission

In an earlier, simple dynamical model for nuclear fission the Hilbert-space was confined to pair excitations. The finite lifetime of the model solution due to the coupling to the orthogonal subspace is estimated treating this coupling in second order perturbation expansion. The numerical value is in agreement with experimental results and shows that the simple model is relevant to nuclear fission. It is discussed how the nucleus forgets the irregular Landau-Zener excitation of the first part of the deformation process and acquires classical features with increasing deformation and excitation energy.     

Spectroscopic effects of conical intersections of molecular potential energy surfaces

A simple vibronic coupling model involving two electronic states and two vibrational modes is considered. The model is based on harmonic diabatic potentials and linear coupling of the diabatic electronic states. It is shown that the adiabatic electronic potential energy surfaces exhibit, in general, a conical intersection. The well known E × E and E × B Jahn-Teller problems are contained as special cases. Using numerical methods the optical absorption spectrum is calculated exactly. Extremely complex vibronic spectra are obtained when the conical intersection occurs within the Franck-Condon (FC) zone. The exact vibronic spectra are compared with spectra calculated in the adiabatic and FC approximation. The genuine spectroscopic effects of conical intersections are revealed by a comparison with the results of standard one-dimensional vibronic coupling calculations. The presence of a conical intersection limits the applicability of the adiabatic and FC approximations much more strongly than in the one-dimensional case. The upper adiabatic electronic state is strongly affected by non-adiabatic coupling even when the point of intersection lies outside the FC zone. The relevance of these results for the calculation of molecular electronic spectra is briefly discussed.     

Determination of the excited state structure of 7-azaindole using a Franck-Condon analysis

The change in structure of 7-azaindole upon electronic excitation was determined by a Franck-Condon analysis of the intensities in the fluorescence emission spectra obtained via excitation of six different vibronic bands. A total of 107 emission band intensities were fit, together with the changes in the rotational constants of four 7-azaindole isotopomers. The geometry change of the ring framework upon electronic excitation from the electronic ground state to the ¹L_b state (ππ*) can be described by an overall expansion of the pyridine ring of 7-azaindole, with minor changes of the pyrrole ring. The resulting geometry changes are interpreted on the basis of ab initio calculations.     

Magnetic transitions in strong coupling expansions for nearly degenerate states

A strong coupling expansion for a two‐band Hubbard model on two sites with nearly degenerate states is considered. A comparative analysis is performed for different schemes of perturbation theory which are applicable to systems with nearly degenerate states. A fourth order approach which builds on a four‐dimensional low‐energy subspace with nearly degenerate states captures accurately the transition from an antiferromagnetic to a ferromagnetic ground state at large on‐site Coulomb interaction.     

Raman Excitation Profile of the G-band Enhancement in Twisted Bilayer Graphene

A resonant Raman study of twisted bilayer graphene (TBG) samples with different twisting angles using many different laser lines in the visible range is presented. The samples were fabricated by CVD technique and transferred to Si/SiO₂ substrates. The Raman excitation profiles of the huge enhancement of the G-band intensity for a group of different TBG flakes were obtained experimentally, and the analysis of the profiles using a theoretical expression for the Raman intensities allowed us to obtain the energies of the van Hove singularities generated by the Moiré patterns and the lifetimes of the excited state of the Raman process. Our results exhibit a good agreement between experimental and calculated energies for van Hove singularities and show that the lifetime of photoexcited carrier does not depend significantly on the twisting angle in the range intermediate angles (?? between 10^° and 15^°). We observed that the width of the resonance window (Γ ≈ 250 meV) is much larger than the REP of the Raman modes of carbon nanotubes, which are also enhanced by resonances with van Hove singularities.     

Structure of the Raman band of the OH stretching vibrations of water and its evolution in a field of second harmonic pulses of a Nd:YAG laser

A fine structure of the Raman band of the OH stretching vibrations of water at 3450 cm^-1 is found upon excitation of the spectra by short trains of second harmonic pulses from a Nd:YAG laser operating at a power of 35 MW/cm² and a pulse repetition frequency of 1 Hz. An increase in the number of pulses in a train from 2 to 128 or in their power leads to the smoothing and asymmetric narrowing of the band and to the shift of its center toward the low-frequency wing, with a subsequent relaxation to the initial state. The observed dynamics of the Raman spectra in the field of the optical pulses is interpreted as manifestation and evolution of a fluctuating network of hydrogen bonds—the destruction and formation of metastable complexes of water having the characteristic frequencies of OH vibrations. Under normal conditions, the lifetime of these complexes in the state induced by the optical field is no less than 1 s.     

Vibrational dynamics and structural investigation of 4‐benzoylpyridine

The polarized Raman spectra in different environments along with the IR counterpart of 4‐benzoylpyridine (4‐BOP) were critically analyzed to assign all of its normal modes of vibration. The knowledge of the positions of different excited electronic states (EESs)was obtained from the study of electronic absorption spectra. Measurement of Raman excitation profiles (REPs)of several normal modes was carried out to get insight into structural and symmetry properties of the molecule. All the experimental observations were substantiated and corroborated theoretically by quantum chemical calculations (QCCs). The possibility of exciton splitting of the ¹L_a band has been explored both from theoretical and experimental points of view. Copyright © 2010 John Wiley & Sons, Ltd.     

Determination of the anion O-3 geometry by Franck-Condon simulations of its photoelectron spectrum

A theoretical method to calculate multidimensional Franck-Condon factors including Duschinsky effects is described and used to simulate the photoelectron spectroscopy of the anion O-3. Geometry optimization and harmonic vibrational frequency calculations have been performed on the (X～)1A1 state of O3 and (X～)2B1 state of O-3. Franck-Condon analyses and spectral simulation were carried out on the first photoelectron band of O-3. The theoretical spectrum obtained by employing CCSD(T)/6-311+G(2d,p) values are in excellent agreement with the observed one. In addition, the equilibrium geometry parameters, re(OO)= 0.135 5±0.000 5 nm and θe(O-O-O) =114.5±0.5°, of the (X～)2B1 state of O-3, are derived by employing an iterative Franck-Condon analysis procedure in the spectral simulation.     

A study of non-thermalized luminescence spectra: the case of Cu2O

The emission spectra of a non-thermalized distribution of excitons in Cu₂O have been studied experimentally and theoretically. The emission spectra were found to exhibit interesting dependence on both the excitation frequencies and on the sample temperatures. These experimental results are explained quantitatively by a simple model calculation of the exciton distribution in Cu₂O under continuous excitation. Using this model the exciton non-radiative lifetime was deduced from the emission spectra. In addition, the present theory accounts for the lineshape of the resonant Raman peaks in Cu₂O more satisfactorily than the existing theory.     

Out‐of‐plane deformations of the heme group in different ferrocytochrome c proteins probed by resonance Raman spectroscopy

We measured the low‐wavenumber polarized resonance Raman spectra of horse heart (hhc), chicken (chc) and yeastC102T (yc) ferrocytochromes c with Soret excitation. We examined the out‐of‐plane (oop) deformations of the heme groups by virtue of relative intensities and depolarization ratios of a variety of oop and in‐plane (ip) Raman active bands. Analysis of relative Raman intensities shows differences in deviation from planarity of the heme groups of yeast, horse heart and chicken cytochromes c. The heme groups in cytochrome c proteins have been shown by normal coordinate static deformation (NSD) analysis from crystal structures to exhibit a dominant ruffling (B_1u) deformation. As a consequence the B_1u modes, γ₁₀ − γ₁₂, become resonance Raman active. We used normalized Raman intensity ratios and depolarization ratios of oop Raman active modes, whose intensities are attributable to specific nonplanar deformations, to estimate and compare their Franck‐Condon‐type and Jahn‐Teller‐type coupling magnitudes for horse heart, chicken and yeast ferrocytochrome c at neutral pH. These coupling magnitudes allow for a quantitative comparison of oop deformations between individual heme groups. Chicken ferrocytochrome was found to have the largest ruffling deformation of the three investigated proteins, followed by horse heart and yeast cytochrome c. The heme group of the former is slightly more ruffled than the corresponding active site of the latter, while saddling in both proteins is substantially larger than in chicken ferrocytochrome c. The Raman data are sensitive enough to allow a comparison of lesser deformations. Doming, which is a kinetic coordinate in many heme proteins, is largest in chicken and smallest in yeast cytochrome c. Waving is largest in yeast, followed by horse heart and chicken cytochrome c. Propellering deformations could be compared for chicken and horse heart cytochrome c and were found to be substantially larger in the latter. A comparison with heme deformations obtained from X‐ray structures (for horse heart and yeast cytochrome c) and from molecular dynamics simulations (MDS) (performed for all three proteins) yields some agreement with the main ruffling and saddling deformations derived from the crystal structures, whereas the heme conformations produced by MDS seem to account better for smaller deformations like doming and propellering. The present study demonstrates the usefulness of resonance Raman spectroscopy for the analysis of nonplanar deformations in heme proteins. Copyright © 2008 John Wiley & Sons, Ltd.