Vibrational Coupling Effects Research Articles

Vibrational Sum-Frequency Spectroscopy of the Water Liquid/Vapor Interface

We present theoretical calculations of the vibrational sum-frequency susceptibility for the water liquid/vapor interface. Our approach builds on previous calculations by us and others, using the time-averaging approximation within the mixed quantum/classical formulation for coupled vibrational chromophores, and electric-field maps for transition frequencies, dipoles, polarizabilities, and intramolecular vibrational couplings. We compare our results for the imaginary part of the susceptibility to those from recent experiments, and comment about the effects of intermolecular vibrational coupling and the assignment of features in the spectrum.

Strong vibrational exciton coupling effects in polarized IR spectra of the hydrogen bond in 2-thiopyridone crystals

This paper presents the results of investigation on polarized IR spectra of the hydrogen bond in 2-thiopyridone crystals. The spectra were measured in the frequency range of the N H and N D bond stretching vibrations, for two different crystalline forms, having developed ab or bc crystal faces. The spectra exhibited extremely strong vibrational exciton coupling effects characterized by a large Davydov- splitting ( correlation field splitting), whose existence was confirmed by a strong difference between the polarized spectra of the two forms of 2-thiopyridone crystals. It was shown that extremely strong exciton interactions involving the translationally non-equivalent hydrogen bonds in the unit cell are responsible for these effects. Isotopic dilution in the crystals caused the disappearance of the spectral effects, ascribed to the inter-dimer exciton couplings, and the simultaneous retaining of the dimeric character of the “ residual” ν N H and ν N D bands. This spectral behavior of the isotopically diluted crystals was interpreted as the result of the dynamical co-operative interactions involving the hydrogen bonds in the lattice. These interactions lead to a non-random distribution of the protons and deuterons in the cyclic hydrogen bond dimeric systems and in consequence to the so-called H/D isotopic “ self- organization” effects in the crystal spectra. The fine structure of the “ residual” ν N H and ν N D bands is also influenced by such non-conventional spectral effects as the selection rule breaking for IR transitions, as well as the “ reversal” exciton coupling effect for centrosymmetric hydrogen bonded dimers. This statement is supported by model calculations of the analyzed band shapes. They are performed in terms of the “ strong- coupling” theory which assumes a strong anharmonic coupling involving different frequency hydrogen bond normal vibrations in the dimers, namely the high-frequency N H stretching and the low-frequency ν N⋯S hydrogen bond stretching vibrations.

Photoinduced Removal of the Franck−Condon Blockade in Single-Electron Inelastic Charge Transmission

A new mechanism of charge transmission through a metal-molecule-metal junction is suggested that is based on optical driving of electronic transitions in the neutral and singly charged molecular state. The effects of strong electron vibrational coupling, intramolecular vibrational energy redistribution, and molecular de-excitation caused by electron-hole pair formation in the leads are taken into account. It is shown that current suppression due to the Franck-Condon blockade can be overcome by opening new transmission channels via photoexcitation.

Comparison of different optical processes of the two-dimensional infrared spectroscopy in the effect of intermolecular vibrational coupling calculated in the time domain: The case of liquid N, N-Dimethylformamide

A time–domain method for calculating two-dimensional infrared (2D-IR) spectra is developed and is applied to the case of the amide I band of liquid N, N-dimethylformamide. This method includes the effects of both the diagonal frequency modulations (of individual molecules in the system) and the off-diagonal (intermolecular) vibrational coupling. The effect of the off-diagonal vibrational coupling on the 2D-IR band profiles is examined. The band profiles for three types of 2D-IR optical processes are compared and discussed in relation to this effect.

Effects of Intermolecular Vibrational Coupling and Liquid Dynamics on the Polarized Raman and Two-Dimensional Infrared Spectral Profiles of Liquid N,N-Dimethylformamide Analyzed with a Time-Domain Computational Method

A time-domain method for calculating polarized Raman and two-dimensional infrared (2D-IR) spectra that includes the effects of both the diagonal frequency modulations (of individual molecules in the system) and the off-diagonal (intermolecular) vibrational coupling is presented and applied to the case of the amide I band of liquid N,N-dimethylformamide. It is shown that the effect of the resonant off-diagonal vibrational coupling and the resulting delocalization of vibrational modes is clearly seen as the noncoincidence effect in the polarized Raman spectrum and some spectral features (especially as asymmetric intensity patterns) in the 2D-IR spectra. The type of 2D-IR spectra (concerning the polarization condition) most appropriate for observing this effect is discussed. On the basis of the agreement between the observed and calculated band profiles of the polarized Raman spectrum, the time dependence of the transient IR absorption anisotropy is also calculated. The method of evaluating the extent of delocalization of vibrational modes that is relevant to the features of these optical signals in the time and frequency domains is discussed. The nature of the molecular motions (concerning the liquid dynamics) that are effective on the diagonal frequency modulations is also examined.

Classical and quantum mechanical infrared echoes from resonantly coupled molecular vibrations

The nonlinear response function associated with the infrared vibrational echo is calculated for a quantum mechanical model of resonantly coupled, anharmonic oscillators at zero temperature. The classical mechanical response function is determined from the quantum response function by setting variant Planck's over 2pi-->0, permitting the comparison of the effects of resonant vibrational coupling among an arbitrary number of anharmonic oscillators on quantum and classical vibrational echoes. The quantum response function displays a time dependence that reflects both anharmonicity and resonant coupling, while the classical response function depends on anharmonicity only through a time-independent amplitude, and shows a time dependence controlled only by the resonant coupling. In addition, the classical response function grows without bound in time, a phenomenon associated with the nonlinearity of classical mechanics, and absent in quantum mechanics. This unbounded growth was previously identified in the response function for a system without resonant vibrational energy transfer, and is observed to persist in the presence of resonant coupling among vibrations. Quantitative agreement between classical and quantum response functions is limited to a time scale of duration inversely proportional to the anharmonicity.

Anharmonic Vibrational Motions In C60: A Potential Energy Surface Derived From Vibrational Self-Consistent Field Calculations

A potential energy surface (PES) is developed for C60 designed to describe vibrational motions valid in the anharmonic limit. The PES is based on a previously existing one that is fit to the harmonic fundamentals and is then modified to generate anharmonicity of all orders and in all terms, but without additional fitted parameters. The resulting Cartesian vibrational motions are decomposed into normal modes, and the anharmonic expansion coefficients are calculated including 2-mode couplings and up to 4th order. The resulting PES is used in a vibrational self-consistent field (VSCF) algorithm to calculate the anharmonically corrected fundamental frequencies. The parameters are then fit to fundamental infrared and Raman frequencies. While it is not possible to assign combination and overtone transitions with sufficient experimental accuracy, conclusions about the effects of anharmonic vibrational coupling in C60 are described.

Electron-molecular vibration coupling effect on the Raman spectrum of organic charge transfer salts

Vibrational spectra of dimerized and tetramerized radical clusters have been calculated to understand the features of electron-molecular vibration (EMV) coupling effects for the charge ordered system (CO). The calculated spectra show that the totally in-phase Raman band, which is usually used as a measure of the molecular ionicity, approaches to the frequency corresponding to the average molecular ionicity in the cluster, with increasing the EMV coupling constant. On the other hand, when large charge disproportionation (CD) presents, vibronic bands show steep shift for small variation of the CD ratio. These results suggest that concerning to the normal modes with a large EMV coupling constant, we should focus on the frequency of the vibronic bands instead of the totally in-phase mode to discuss the charge distribution.

Interference, dephasing, and vibrational coupling effects between coherence pathways in doubly vibrationally enhanced nonlinear spectroscopies

Doubly vibrationally enhanced (DOVE) four-wave mixing is capable of producing coherent two-dimensional (2D) vibrational spectra with cross-peaks that reflect intramolecular and intermolecular mode coupling. These methods are based on double vibrational coherences that are analogous to the double spin coherences in 2D nuclear magnetic resonance methods. In particular, there are two pathways for DOVE infrared spectroscopy that interfere to change the line shapes of doubly resonant cross-peaks. The interference depends on the relative dephasing rates of the coherences involved in the different pathways. We derive relationships between the dephasing rates of the different coherences in DOVE pathways and show how these relationships affect the line shapes of DOVE resonances. Comparisons are made with experimental 2D DOVE spectra.

Ab Initio Study of Vibrational Anharmonic Coupling Effects in Oligo(para-phenylenes)

ABSTRACTIn this study we present a theoretical approach to simulate vibrational anharmonic coupling effects seen in the Raman spectra of oligo(para-phenylenes). Quantum chemical ab inito methods are applied to determine anharmonic force constants and energy corrections on the harmonic vibrational frequencies of the isolated molecules. Semiempirical methods are applied to compute Raman intensities of fundamentals and combination bands. This methodology is then used to characterize a previously unassigned Fermi resonance around 1600 cm-1. The evolution of this quantum mechanical resonance with oligomer length and planarity is compared to experimental data.

Electronic correlation and electron–molecular vibration coupling effects on non-linear optical properties of molecular dimer

Molecular dimer with two radical electrons is considered as a model system to study the effects of electronic correlation and electron–intramolecular vibration coupling on the non-linear optical properties. The static polarizability α and the static second hyperpolarizability γ are calculated and their dependencies on the model parameters are analyzed. The ion–radical compounds with large and unsymmetrical molecules are shown to have a potential for a high non-linearity.

Vibrational and rotational coupling effects in the direct scattering of H 2 from Cu(111)

We investigate how the coupling between molecular vibration and rotation affects the direct scattering of H 2 from Cu(111) by performing coupled channel calculations. Our calculation results show that the rotational excitation probability of H 2 scattered (vibrationally elastic) in the first vibrationally excited state ( ν=1) increases more rapidly than that of H 2 scattered (vibrationally elastic) in the vibrational ground state ( ν=0) with increasing translational energy in the low translational energy region. Furthermore, the calculation results for the ratio of the probability of H 2 scattered in ν=1 and rotational state J=2 to the probability of H 2 scattered in ν=1 and J=0 as a function of translational energy qualitatively reproduce the experimental results in the low translational energy region.

Rotational and Vibrational Coupling Effects on the Dissociative Adsorption and Associative Desorption Dynamics of D2/Cu(111)

We investigate and discuss the effects of the coupling between molecular rotation and vibration on the dissociative adsorption and associative desorption dynamics of D 2 /Cu(111), with the aid of a model Hamiltonian and the concept of a reaction path. In our model Hamiltonian, we take into account the molecular orientation dependence of the reaction path curvature in the kinetic energy term. We calculate for the dissociative adsorption probability of D 2 as a function of its initial translational energy, vibrational and rotational states, using the coupled channel method. By invoking the principle of microscopic reversibility, we obtain the corresponding associative desorption probability results for D 2 as a function of its final translational energy, vibrational and rotational states from the dissociative adsorption probability results. Furthermore, we show that the calculation results qualitatively agree with experimental results.

An extension of CVDV model to Zeldovich exchange reactions for hypersonic non-equilibrium air flows

A new preferential vibration-dissociation-exchange reactions coupling model – labelled CVDEV – resulting from an extension of the well-known Treanor and Marrone CVDV model, has been derived to take into account the coupling between the vibrational excitation of the $N_{2}$ and $O_{2}$ molecules and the two Zeldovich exchange reactions. Analytical expressions for the exchange reactions coupling factor and for the average vibrational energy lost – or gained – by a molecule through an exchange reaction have been developed. The influence of such a coupling has been shown by means of numerical simulations of hypersonic air flows through normal and bow shock waves. Code-to-code comparisons between our model and other recent approaches have been conducted. The infrared radiation of nitric oxide behind a normal shock wave resulting from computations with the CVDEV model has been compared with other coupling model results and to recent shock tube experimental data. These comparisons have shown a good agreement of our model results with the experimental data. In this context, the results show the prominent influence of $N_{2}$ vibration coupling on the first Zeldovich reaction, and the absence of $O_{2}$ vibration coupling effects on the second Zeldovich reaction.

ChemInform Abstract: Vibrational Coupling Effects in the Resonance Raman Spectra of O2 Adducts of Heme Proteins and Model Compounds

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Infrared Spectroscopy in Electrochemistry: New Methods and Connections to UhV Surface Science

This review discusses recent advances in the use of infrared spectroscopy in electrochemistry. The central focus is on analytical factors that affect the ability to derive structural information from infrared spectra of molecular adlayers. The effects of vibrational coupling and dielectric screening are emphasized. Applications of real-time polarization modulation and step-scan Fourier transform infrared spectroscopy are also presented.

Vibrational coupling effects in the resonance Raman spectra of O2 adducts of heme proteins and model compounds

Vibrational coupling effects in the resonance Raman spectra of O2 adducts of heme proteins and model compounds

Kinetics of nitric oxide formation behind shock waves

The infrared radiation of nitric oxide (NO) behind a shock wave in O2-N2 mixtures has been calculated by two different techniques, and compared with recent shock-tube experiments. The first technique (model I) utilizes the Park model. This model incorporates the vibrational relaxation of O2 and N2 and assumes a Boltzmann distribution of vibrational energy during the relaxation process. Model II uses a master equation solution, employing recently published state-to-state vibration-translation and vibration-vibration transition probabilities. Vibration-chemistry coupling is provided through the MacheretFridman-Rich model (MFR). The calculations are compared with experimental results for shock waves in the range of 3-4 km/s. Results of the two model calculations are compared at speeds up to 9 km/s, for both normal shocks and bow shocks. The two models predict nearly the same NO production rates behind all of the normal shocks, and show the prominent effect of N2 vibrational coupling in the reaction N2 + O —> NO + N. For high-altitude bow shocks, where extreme vibrational nonequilibrium is present, there are large differences in the results calculated by the Park and MFR coupling techniques.

Two-photon optical bistability in polar molecules: vibrational coupling effects

Single-beam two-photon optical bistability in a two-level system where the levels have unequal electric dipole moments is analyzed theoretically. By means of unitary transformations the system is transformed into a two-level nonpolar system with a field-dependent effective transition dipole moment. Two-photon optical bistability is found to be favored as the difference between the dipole moments grows. When the excited electronic system is coupled to a vibrational normal mode of the nuclei, the susceptibility becomes three-valued and consequently a different type of optical bistability appears, which can coexists with the previous one.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Vibrational overtone spectroscopy and coupling effects in monolayer CO on NaCl(100)

Monolayer physisorbed CO on NaCl(100) single crystal surfaces has been investigated with polarized Fourier transform infrared (FTIR) spectroscopy at 5 K. While the fundamental ( v = 1 ← v = 0) transition of the monolayer has been studied previously, we report for the first time the overtone ( v = 2 ← v = 0) transition. Its integrated absorbance is very weak, being only 0.5% of the fundamental. Comparisons of the transition frequencies, the bandwidths and absorbances for these two transitions together with those of dilute isotopes allow us to isolate the contribution of dynamic and static coupling effects of molecules within the monolayer. We can also distinguish homogeneous and heterogeneous effects from band profiles of these various transitions. The method of analysis of these data is the exciton model appropriate to treating the coupling of the electric multipoles of molecules within the adlayer.