Franck-Condon Mechanism Research Articles

Can the Resonance Raman Optical Activity Spectrum Display Sign Alternation?

The monosignate character of resonance Raman optical activity (RROA) spectra has been often taken as granted in experimental and computational approaches, on the basis of basic theoretical approximations only considering resonance with a single electronic state of the molecule and the scattering process to be governed by the Franck-Condon mechanism. We show in this letter for the first time that, by resorting to a fully quantum mechanical (QM) methodology able to take into account all terms entering the general definition of RROA, and which considers excited state interference and Herzberg-Teller effects, sign alternation and at the same time intensity enhancement in RROA spectra is obtained. Such features constitute an important milestone toward the exploration of RROA of a wide range of chiral biological molecules.

Band gap tuning and orbital mediated electron–phonon coupling in HoFe1−xCrxO3 (0 ≤ x ≤ 1)

We report on the evidenced orbital mediated electron–phonon coupling and band gap tuning in HoFe1−xCrxO3 (0 ≤ x ≤ 1) compounds. From the room temperature Raman scattering, it is apparent that the electron-phonon coupling is sensitive to the presence of both the Fe and Cr at the B-site. Essentially, an Ag like local oxygen breathing mode is activated due to the charge transfer between Fe3+ and Cr3+ at around 670 cm−1, this observation is explained on the basis of Franck-Condon mechanism. Optical absorption studies infer that there exists a direct band gap in the HoFe1−xCrxO3 (0 ≤ x ≤ 1) compounds. Decrease in band gap until x = 0.5 is ascribed to the broadening of the oxygen p-orbitals as a result of the induced spin disorder due to Fe3+ and Cr3+ at B-site. In contrast, the increase in band gap above x = 0.5 is explained on the basis of the reduction in the available unoccupied d-orbitals of Fe3+ at the conduction band. We believe that above results would be helpful for the development of the optoelectronic devices based on the ortho-ferrites.

Lattice dynamics of theαandβphases of LiFe5O8

The polarized Raman spectra of the inverse spinel LiFe${}_{5}$O${}_{8}$ (LFO) were studied for both B-site-ordered ($\ensuremath{\alpha}$) and B-site-disordered ($\ensuremath{\beta}$) phases. The symmetry of all the Raman lines was determined and an assignment to phonon modes involving specific atomic motions was done by comparison to lattice dynamics calculations. The spectra exhibit strongly resonant behavior, the enhancement of some one-phonon bands and all the two-phonon bands being probably related to the Franck-Condon mechanism of excitation and decay of the electronic d-d transition of Fe${}^{3+}$. The disordered $\ensuremath{\beta}$ phase, obtained by rapid quenching after postannealing at a temperature above the nominal order-disorder transition, coexists with remnants of the $\ensuremath{\alpha}$ phase.

Properties of Hydrogen Terminated Diamond as a Photocathode

Electron emission from the negative electron affinity (NEA) surface of hydrogen terminated, boron doped diamond in the [100] orientation is investigated using angle resolved photoemission spectroscopy (ARPES). ARPES measurements using 16 eV synchrotron and 6 eV laser light are compared and found to show a catastrophic failure of the sudden approximation. While the high energy photoemission is found to yield little information regarding the NEA, low energy laser ARPES reveals for the first time that the NEA results from a novel Franck-Condon mechanism coupling electrons in the conduction band to the vacuum. The result opens the door to the development of a new class of NEA electron emitter based on this effect.

ChemInform Abstract: Resonance Raman and Absorption Spectroscopy of the Lowest Triplet State of 1,3,5-Hexatriene and Deuteriated Derivatives at 183 K; Molecular Structure in the T1 State

Time-resolved resonance Raman (RR) spectra of the lowest excited triplet state T1 of (E)-1,3,5-hexatriene, (E)-3-deuterio-1,3,5-hexatriene, and (E)-1,1-dideuterio-1,3,5-hexatriene are obtained at 183 K. The T1 potential energy surface (PES), determining energy minima, equilibrium geometries, frequencies, and normal modes of vibration, is calculated, and RR intensities are obtained on the basis of the Franck-Condon mechanism. By comparison with the calculated spectra, the observed RR spectra are assigned to an equilibrium mixture of planar E and Z species in the T1 state. The relative intensity of the E and Z bands at temperatures between 293 and 183 K gives an upper limit of 0.22 kcal/mol for the energy difference between the planar E and Z forms. Time-resolved triplet-triplet absorption measurements as a function of temperature between 296 and 183 K yield a frequency factor A of 1.6 ¥ 107 s-1 and an apparent activation energy Ea of 0.7 kcal/mol for the decay of the T1 state. Consequently, an upper limit of 0.7 kcal/mol above the planar E and Z geometries is established for the possible local energy minimum at the twisted geometry. On the basis of these data, a PES is proposed for the T1 state of 1,3,5-hexatriene with respect to torsion around the central CC bond and the C2C3 and C4C5 bonds.

Franck-Condon higher order lattice excitations in theLaFe1−xCrxO3(x=0, 0.1, 0.5, 0.9, 1.0) perovskites due to Fe-Cr charge transfer effects

First and higher order lattice excitations in the $B$-site disordered perovskites $\mathrm{La}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3}$ ($x=0$, 0.1, 0.5, 0.9, and 1) and ${\mathrm{La}}_{0.835}{\mathrm{Sr}}_{0.165}{\mathrm{Fe}}_{0.5}{\mathrm{Cr}}_{0.5}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ are investigated using temperature dependent and polarized inelastic light scattering [$\ensuremath{\lambda}=515\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ $(2.41\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and $676\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ $(1.83\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$] on oriented crystallites. A peak at approximately $2.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the imaginary part of the dielectric function of $\mathrm{La}{\mathrm{Fe}}_{0.5}{\mathrm{Cr}}_{0.5}{\mathrm{O}}_{3}$ is assigned to a charge transfer from ${\mathrm{Fe}}^{3+}$ $({d}^{5})$ to ${\mathrm{Cr}}^{3+}$ $({d}^{3})$ ions, coupled with the appearance of an intense ${A}_{g}$-like mode at approximately $700\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in the Raman data. This excitation is identified as a symmetric oxygen breathing mode activated by the Fe-Cr charge transfer through an orbital coupling mechanism. Higher order scattering (up to seventh order) of the intrinsic Raman active symmetric breathing mode is also explained by an orbital-mediated electron-phonon coupling, similar to the Franck-Condon effect observed in the Jahn-Teller active-perovskite-structured manganite $\mathrm{La}\mathrm{Ma}{\mathrm{O}}_{3}$. These results show that the Franck-Condon mechanism is a more common mechanism for resonant higher order scattering in solids than previously believed and propose the $\mathrm{La}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3}$ system as a model system for electron-phonon coupling and higher order Raman scattering in solids.

Low-temperature mixed spin state of Co3+ in LaCoO3 evidenced from Jahn–Teller lattice distortions

One-phonon and multiphonon excitations of the single-crystalline LaCoO3 were studied using Raman spectroscopy in the temperature region 5–300K. First-order Raman spectra show a larger number of phonon modes than allowed for the rhombohedral structure. Additional phonon modes are interpreted in terms of activated modes due to lattice distortions, arising from the Jahn–Teller (JT) activity of the intermediate-spin (IS) state of the Co3+ ions. In particular, the 608-cm−1 stretching-type mode shows anomalous behavior in peak energy and scattering intensity as a function of temperature. The anomalous temperature dependence of the second-order phonon excitations spectra is in accordance with the Franck–Condon mechanism that is characteristic for a JT orbital order.

Surface-enhanced Raman scattering and fluorescence near metal nanoparticles

We present a general model study of surface-enhanced resonant Raman scattering and fluorescence, focusing on the interplay between electromagnetic (EM) effects and the molecular dynamics as treated by a density matrix calculation. The model molecule has two electronic levels, is affected by radiative and non-radiative damping mechanisms, and a Franck-Condon mechanism yields electron-vibration coupling. The coupling between the molecule and the electromagnetic field is enhanced by placing it between two Ag nanoparticles. The results show that the Raman scattering cross section can, for realistic parameter values, increase by some 10 orders of magnitude (to $\sim10^{-14}$ cm$^2$) compared with the free-space case. Also the fluorescence cross section grows with increasing EM enhancement, however, at a slower rate, and this increase eventually stalls when non-radiative decay processes become important. Finally, we find that anti-Stokes Raman scattering is possible with strong incident laser intensities, $\sim 1$ mW/$\mu$m.

Unified Treatment of Fluorescence and Raman Scattering Processes near Metal Surfaces

We present a general model study of surface-enhanced resonant Raman scattering and fluorescence focusing on the interplay between electromagnetic effects and the molecular dynamics. Our model molecule is placed close to two Ag nanoparticles and has two electronic levels. A Franck-Condon mechanism provides electron-vibration coupling. Using realistic parameter values for the molecule we find that an electromagnetic enhancement by 10 orders of magnitude can yield Raman cross sections sigma(R) of the order 10(-14) cm(2). We also discuss the dependence of sigma(R) on incident laser intensity.

Orbital ordering in LaMnO3 investigated by resonance Raman spectroscopy.

Orbital ordering leads to an unconventional excitation spectrum that we investigate by resonance Raman scattering using incident photon energies between 1.7 and 5.0 eV. We use spectral ellipsometry to determine the corresponding dielectric function. Our results show resonant behavior of the phonon Raman cross section when the laser frequency is close to the orbiton-excitation energy of 2 eV in LaMnO3. We show an excellent agreement between theoretical calculations based on the Franck-Condon mechanism activating multiphonon Raman scattering in first order of the electron-phonon coupling and the experimental data of phonons with different symmetries.

Multiphonon resonant Raman scattering predicted inLaMnO3from the Franck-Condon process via self-trapped excitons

Resonant behavior of the Raman process is predicted when the laser frequency is close to the orbital excitation energy of LaMnO3 at 2 eV. The incident photon creates a vibrationally excited self-trapped ``orbiton'' state from the orbitally-ordered Jahn-Teller (JT) ground state. Trapping occurs by local oxygen rearrangement. Then the Franck-Condon mechanism activates multiphonon Raman scattering. The amplitude of the $n$-phonon process is first order in the electron-phonon coupling $g$. The resonance occurs {\it via} a dipole forbidden $d$ to $d$ transition. We previously suggested that this transition (also seen in optical reflectivity) becomes allowed because of asymmetric oxygen fluctuations. Here we calculate the magnitude of the corresponding matrix element using local spin-density functional theory. This calculation agrees to better than a factor of two with our previous value extracted from experiment. This allows us to calculate the absolute value of the Raman tensor for multiphonon scattering. Observation of this effect would be a direct confirmation of the importance of the JT electron-phonon term and the presence of self-trapped orbital excitons, or ``orbitons''.

The Resonance Raman Spectra Of Biologically Relevant Polyene Derivatives in T1: A Theoretical Investigation

The vibrational structure of the T1 resonance Raman spectra of β-ionylidene acetaldehyde (C15 aldehyde) and retinal are analyzed with the help of semiempirical quantum chemical calculations. The T1 equilibrium structures and vibrational force fields, along with the structure in the Tn state which is in resonance with the exciting wavelength, are obtained and employed to model the spectra under the assumption of a dominant Franck–Condon mechanism.

The T1 resonance Raman spectra of biochemically relevant chromophores a theoretical investigation

The vibrational structure of the T 1 resonance Raman spectra of hexatriene, β-ionylidene acetaldehyde (C15 aldehyde) and β-carotene are analyzed with the help of semiempirical and ab initio quantum chemical calculations. The T 1 equilibrium structures and vibrational force fields of the mono- cis- and trans-isomeric species are obtained and employed to model the spectra under the assumption of a dominant Franck-Condon mechanism. The simulations are used to identify the species that contribute to the observed spectra and to map the shape of the T 1 potential energy surface.

The Raman and vibronic activity of intermolecular vibrations in aromatic-containing complexes and clusters

Theoretical and experimental results pertaining to the excitation of intermolecular vibrations in the Raman and vibronic spectra of aromatic-containing, weakly bound complexes and clusters are reported. The theoretical analysis of intermolecular Raman activity is based on the assumption that the polarizability tensor of a weakly bound species is given by the sum of the polarizability tensors of its constituent monomers. The analysis shows that the van der Waals bending fundamentals in aromatic–rare gas complexes may be expected to be strongly Raman active. More generally, it predicts strong Raman activity for intermolecular vibrations that involve the libration or internal rotation of monomer moieties having appreciable permanent polarizability anisotropies. The vibronic activity of intermolecular vibrations in aromatic-rare gas complexes is analyzed under the assumption that every vibronic band gains its strength from an aromatic-localized transition. It is found that intermolecular vibrational excitations can accompany aromatic-localized vibronic excitations by the usual Franck–Condon mechanism or by a mechanism dependent on the librational amplitude of the aromatic moiety during the course of the pertinent intermolecular vibration. The latter mechanism can impart appreciable intensity to bands that are forbidden by rigid-molecule symmetry selection rules. The applicability of such rules is therefore called into question. Finally, experimental spectra of intermolecular transitions, obtained by mass-selective, ionization-detected stimulated Raman spectroscopies, are reported for benzene–X (X=Ar, –Ar2, N2, HCl, CO2, and –fluorene), fluorobenzene–Ar and –Kr, aniline–Ar, and fluorene–Ar and –Ar2. The results support the conclusions of the theoretical analyses and provide further evidence for the value of Raman methods in characterizing intermolecular vibrational level structures.

Thermal energy reactions of OH − + Cl 2, Br 2: rate coefficients, product branching fractions and OH product vibrational populations

The rate coefficients, branching fractions, and OH product vibrational distributions are measured in a flowing afterglow apparatus for the OH − + Cl 2 and OH − + Br 2 reactions at thermal energy. Both reactions proceed at the Langevin collision rate, with k = 1.33(±0.13) × 10 −9 cm 3 molecule −1 s −1 for OH − Cl 2 and k = 1.60(±0.16) × 10 −9 cm 3 molecule −1 s −1 for OH − + Br 2. The branching fractions for OH − + Cl 2 are 0.75(±0.10) Cl − 2 + OH, 0.24(±0.10) Cl − + HOCl and a discernible ⩽0.01 HOCl − + Cl. For OH − + Br 2, the branching fractions are 0.75(±0.10) Br − 2 + OH, 0.15(±0.05) Br − + HOBr and 0.10(±0.05) HOBr − + Br. The vibrational populations of the OH product from the charge transfer channel are measured by laser-induced fluorescence detection and found to be OH( v = 1)/OH( v = 0) = 0.01(±0.01) for both reactions. While the OH( v) states are consistent with a Franck—Condon mechanism, arguments for a long-lived intermediate complex are made on the basis of substantial branching fractions into Cl − + HOCl (Br − + HOBr) and HOCl − + Cl (HOBr − + Br) channels and the extreme difference in bond length for Cl 2 and Cl − 2 (Br 2 and Br − 2).

Resonance Raman and absorption spectroscopy of the lowest triplet state of 1,3,5-hexatriene and deuteriated derivatives at 183 K: molecular structure in the T1 state

Time-resolved resonance Raman (RR) spectra of the lowest excited triplet state T1 of (E)-1,3,5-hexatriene, (E)-3-deuterio-1,3,5-hexatriene, and (E)-1,1-dideuterio-1,3,5-hexatriene are obtained at 183 K. The T1 potential energy surface (PES), determining energy minima, equilibrium geometries, frequencies, and normal modes of vibration, is calculated, and RR intensities are obtained on the basis of the Franck-Condon mechanism. By comparison with the calculated spectra, the observed RR spectra are assigned to an equilibrium mixture of planar E and Z species in the T1 state. The relative intensity of the E and Z bands at temperatures between 293 and 183 K gives an upper limit of 0.22 kcal/mol for the energy difference between the planar E and Z forms. Time-resolved triplet-triplet absorption measurements as a function of temperature between 296 and 183 K yield a frequency factor A of 1.6 ¥ 107 s-1 and an apparent activation energy Ea of 0.7 kcal/mol for the decay of the T1 state. Consequently, an upper limit of 0.7 kcal/mol above the planar E and Z geometries is established for the possible local energy minimum at the twisted geometry. On the basis of these data, a PES is proposed for the T1 state of 1,3,5-hexatriene with respect to torsion around the central CC bond and the C2C3 and C4C5 bonds.

The resonance Raman effect of dithiocroconate

Dithiocroconate (anion of 4-cyclopentene-1,2,3-trione-4,5-dimercapto) was investigated by resonance Raman spectroscopy. Excitation in resonance with the first allowed transition at ca. 465 nm causes the enhancement of virtually all the observed modes, indicating the delocalized nature of the chromophore. On the other hand, the fact that all the enhanced modes are totally symmetrical indicates that in contrast to its parent ion (croconate), dithiocroconate derives its resonance enhancement from the Franck-Condon mechanism. Comparison of the Raman frequency values for croconate and dithiocroconate reveals that for the latter, electron delocalization in the ring, although it is present, is not as symmetrical as in the former.

On the Franck–Condon behavior of the H+Cl2 reaction

Extended quantum collinear calculations of the H+Cl2 reaction have been compared with model results in order to single out distinctive properties of reactive probabilities for a reaction thought to be governed by a Franck–Condon mechanism. Such an analysis has shown that even a simple model is able to rationalize the Franck–Condon contributions to the product vibrational distribution when the energy dependence of the exact quantum results is taken into account properly. On top of that, it has been shown that the product vibrational distribution of a Franck–Condon driven reaction need not have v+1 peaks (v being the initial vibrational state of reactants).

Laser probing of product-state distributions in thermal-energy ion–molecule reactions

Vibrational and rotational product-state distributions are determined for thermal-energy charge-transfer reactions and Penning ionization processes using laser-induced fluorescence detection in both a flowing afterglow apparatus and a single-collision molecular beam device. The reactions investigated are the charge transfers between N++ CO, Ar++ N2, Ar++ CO, and the Penning ionization of N2 by Ne(3P2). Vibrational distributions provide direct information on major features of the dynamics, such as whether a Franck–Condon mechanism is dominant, whether collision complex formation is important, or if selective vibrational passageways exist between the electronic potential-energy surfaces. The rotational distributions show a variety of additional discriminating dynamical effects, including corroborating evidence for Franck–Condon channels, pinpointing separate mechanisms for different vibrational product states and detecting microscopic bimodalities within individual vibrational levels, which are indicative of multiple entrance- or exit-channel pathways.

Product vibrational state distributions of thermal energy charge transfer reactions determined by laser-induced fluorescence: N++CO→CO+(v=0–2)+N

The nascent vibrational state distribution of the N++CO→CO+(v=0–2)+N charge transfer reaction is measured at thermal energy. The reaction is carried out in a flowing afterglow and the vibrational state populations are determined by laser-induced fluorescence on the CO+(A 2Π−X 2Σ+) system. The nascent vibrational state distribution for the N++ CO reaction is (0.71±0.05)v=0:(0.27±0.04)v=1:(0.02±0.01)v=2. The observed vibrational distribution suggests that neither a long-range Franck–Condon mechanism nor an energy resonant process adequately describes the charge transfer reaction. A dual channel mechanism of the reaction is considered, in which a fraction of the reactive collisions proceed by a long-range Franck–Condon mechanism while the remainder proceed via a long-lived NCO+ intermediate. The intermediate may lead to the observed extent of CO+ vibrational excitation either through statistical partitioning of the energy or by dynamical changes in the CO bond length through specific molecular orbital occupancies.