Vibron Phonon Research Articles

Intense deep-red zero phonon line emission of Mn4+ in double perovskite La4Ti3O12.

Phosphors that emit in the deep-red spectral region are critical for plant cultivation light-emitting diodes. Herein, ultrabroadband deep-red luminescence of Mn4+ in La4Ti3O12 was studied, which showed intense zero phonon line emission. The double-perovskite structural La4Ti3O12 simultaneously contains two Ti4+ sites forming slightly- and highly-distorted TiO6 octahedra, respectively. The influence of octahedral distortion on the Mn4+ emission energy in the two distinct Ti4+ sites was studied both experimentally and theoretically. The spectral measurements indicated that Mn4+ in La4Ti3O12 showed intense zero phonon line emission (ZPL) at deep-red 710-740 nm under excitation of 400 nm charging the O2-→ Mn4+ charge transfer transition. The splitting of the ZPL of the Mn4+ 2Eg→4A2g transition as well as the intensity of ZPL relative to the vibronic phonon sideband emissions were found to be greatly influenced by the degree of octahedral distortion. The crystal-field strength and Racah parameters of Mn4+ in each Ti4+ site were also estimated. The Mn4+ 2Eg→4A2g luminescence exhibited severe thermal quenching, which was explained by the low-lying 4T2g level and charge-transfer state.

Shape, orientation and magnitude of the curl quantum flux, the coherence and the statistical correlations in energy transport at nonequilibrium steady state

We provide a quantitative description of the nonequilibriumness based on the model of coupled oscillators interacting with multiple energy sources. This can be applied to the study of vibrational energy transport in molecules. The curl quantum flux quantifying the nonequilibriumness and time-irreversibility is quantified in the coherent representation and we find the geometric description of the shape and polarization of the flux which provides the connection between the microscopic description of quantum nonequilibriumness and the macroscopic observables, i.e., correlation function. We use the Wilson loop integral to quantify the magnitude of curl flux, which is shown to be correlated to the correlation function as well. Coherence contribution is explicitly demonstrated to be non-trivial and to considerably promote the heat transport quantified by heat current and efficiency. This comes from the fact that coherence effect is microscopically reflected by the geometric description of the flux. To uncover the effect of vibron–phonon coupling between the vibrational modes of molecular stretching (vibron) and the molecular chain (phonon), we further explore the influences of localization, which leads to the coherent and incoherent regimes that are characterized by the current–current correlations.

Vibrons in finite size molecular lattices: a route for high-fidelity quantum state transfer at room temperature

A communication protocol is proposed in which vibron-mediated quantum state transfer takes place in a molecular lattice. We consider two distant molecular groups grafted on each side of the lattice. These groups form two quantum computers where vibrational qubits are implemented and received. The lattice defines the communication channel along which a vibron delocalizes and interacts with a phonon bath. Using quasi-degenerate perturbation theory, vibron–phonon entanglement is taken into account through the effective Hamiltonian concept. A vibron is thus dressed by a virtual phonon cloud whereas a phonon is clothed by virtual vibronic transitions. It is shown that three quasi-degenerate dressed states define the relevant paths followed by a vibron to tunnel between the computers. When the coupling between the computers and the lattice is judiciously chosen, constructive interference takes place between these paths. Phonon-induced decoherence is minimized and a high-fidelity quantum state transfer occurs over a broad temperature range.

Vibron phonon in a lattice of H-bonded peptide units: A criterion to discriminate between the weak and the strong coupling limit

Based on dynamical considerations, a simple and intuitive criterion is established to measure the strength of the vibron-phonon coupling in a lattice of H-bonded peptide units. The main idea is to compare separately the influence of both the vibron-phonon coupling and the dipole-dipole interaction on a specific element of the vibron reduced density matrix. This element, which refers to the coherence between the ground state and a local excited amide-I mode, generalizes the concept of survival amplitude at finite temperature. On the one hand, when the dipole-dipole interaction is neglected, it is shown that dephasing-limited coherent dynamics is induced by the vibron-phonon coupling. On the other hand, when the vibron-phonon coupling is disregarded, decoherence occurs due to dipole-dipole interactions since the local excited state couples with neighboring local excited states. Therefore, our criterion simply states that the strongest interaction is responsible for the fastest decoherence. It yields a critical coupling chi( *) approximately 25 pN at biological temperature.

Two-vibron bound states in alpha-helix proteins: the interplay between the intramolecular anharmonicity and the strong vibron-phonon coupling.

The influence of the intramolecular anharmonicity and the strong vibron-phonon coupling on the two-vibron dynamics in an alpha-helix protein is studied within a modified Davydov model. The intramolecular anharmonicity of each amide-I vibration is considered, and the vibron dynamics is described according to the small polaron approach. A unitary transformation is performed to remove the intramolecular anharmonicity, and a modified Lang-Firsov transformation is applied to renormalize the vibron-phonon interaction. Then a mean field procedure is realized to obtain the dressed anharmonic vibron Hamiltonian. It is shown that the anharmonicity modifies the vibron-phonon interaction, which results in an enhancement of the dressing effect. In addition, both the anharmonicity and the dressing favor the occurrence of two different bound states whose properties strongly depend on the interplay between the anharmonicity and the dressing. This dependence was summarized in a phase diagram which characterizes the number and the nature of the bound states as a function of the relevant parameters of the problem. For a significant anharmonicity, the low-frequency bound states describe two vibrons trapped onto the same amide-I vibration, whereas the high-frequency bound states refer to the trapping of the two vibrons onto nearest neighbor amide-I vibrations.

Temperature Dependence of the Raman Shift and Raman Width of Solid Parahydrogen: Effect of Vibron–Phonon Scattering

We present a model for vibron–phonon scattering in molecular crystals. A general kinetic equation for the vibrons is derived. We use this equation to calculate the frequency shift and dephasing rate of the vibrons as functions of temperature. We find good agreement between the calculated results and the experimental observations for the Raman shift and Raman width of the fundamental vibrational Raman transition of solid parahydrogen.

Doping created vibronic phonon softening channel in cuprates

Vibronic interband dynamical destabilization factor for a two-band superconductor with an itinerant and a doping-created band (cuprate) is calculated. The phonon renormalization becomes maximal at the under/optimal doping separation line, where the bands begin to overlap. Experimentally this line is connected with a structural transformation which deforms CuO 2 planes.

DNA dynamics and endogeneous fields

In this paper we investigate the influence of endogeneous AC fields, generated by a living cell itself, on DNA dynamics. Both quantum nonlinear vibron–phonon interaction model and a model of a highly localized excitation are used. The transition of a broad solitonic excitation into a highly localized one affected by AC fields is calculated using Kubo’s formalism for linear response of the system.

Infrared Studies of Ar(H2)2 at Low Temperature and High Pressure

An extensive infrared absorption study of the compound Ar(H2)2in the range of pressure from 5 to 30 GPa, and between 30 K and room temperature is presented. This study has permitted the identification and assignment of high–frequency and low frequency vibron–phonon combination bands, as well as of the roto–vibrational bands. Spectra changes lowering temperature, with the appearance of the Q1(0) vibrational peak, are interpreted as arising from conversion of ortho–hydrogen into para–hydrogen in the solid.

Transform theory for resonant Raman spectra of impurity crystals

A general relation between the Raman and absorption spectra of an impurity center is found. This relation takes into account the dependence of both spectra on Herzberg—Teller and Franck—Condon interactions. The one-quantum and vibron—phonon spectra are analyzed. It is found that only one-quantum Raman peaks for symmetrical modes can be expressed via the absorption band of an impurity center. This relation is violated if we consider one-quantum Raman peaks for non-symmetrical modes. These peaks cannot be expressed via the total absorption band. Numerical calculations of the vibron—phonon resonant Raman spectra are carried out. A sharp dependence of these spectra on the frequency ω a of exciting light is analyzed.

Vibron–phonon excitations in the molecular crystals N2, O2, and CO by Fourier transform infrared and Raman studies

The influence of temperature on the vibron–phonon combination band in the Raman and infrared (ir) spectra of the N2 and O2 molecular crystals supports the possibility of an assignment of the main features in the sideband to strong contributions from translational and librational phonons in points of high symmetry in the reciprocal lattices. The temperature behavior in Raman and ir sideband spectra in α-N2 is attributed to distinct anharmonicities in the isotropic and anisotropic parts of the potential and to different coupling mechanisms, resulting in a librational and translational weighted one-phonon density of states (DOS). In contrast, such an interpretation is not feasible for the poorly structured CO sideband, although the crystal structures of CO and N2 are nearly identical. The difference is attributed to strong anharmonicities and the presence of a weak dipole moment in the former, which introduces strong lattice mode coupling. Crossing the α–β phase transition has marked effects on the sidebands (shape, intensity) in both N2 and O2 crystals, which reflects the orientational disorder in the β phase of the former and the importance of the change in magnetic interactions in the latter. For both molecular crystals, the vibron–phonon coupling to the electromagnetic field is stronger in the ir than in the Raman spectra and seems predominantly due to electrical anharmonicities.

The vibron–phonon sidebands in the Fourier-transform infrared spectra of the molecular crystal CO2

FTIR investigations on polycrystalline CO2 at various temperatures show Stokes and anti-Stokes sidebands coupled to the vibrational modes v3, v2, and v+/v− (Fermi dyad). The latter is infrared forbidden and may be eliminated from spectra of good quality polycrystalline films. The main peaks in the structure of the sidebands may be assigned to phonons from points of highest symmetry in the Brillouin zone. The vibron–phonon coupling process is mode specific and driven by electrical anharmonicity in case of v+ and v−. For the infrared allowed fundamentals, and especially at the Γ point, the coupling process is also driven by mechanical anharmonicity. The dispersion of these internal modes at k=0 influences the coupled phonons and thus the one phonon density of states is correctly mirrored by the v+ and v− sidebands only.

Raman spectroscopy of matrix-isolated hydrogen. II. Influence of defects on matrices

The hydrogen molecule is treated as a model substance for matrix-isolation effects. Raman investigations are made concerning the influence of the defect on the matrix material Ar, Kr, Xe, N2, and O2. The influence of the matrix on the hydrogen impurities is investigated in the preceding paper.A detailed comparison of the spectra of pure molecular crystals with the spectra of doped crystals (1% H2 or 1% D2) shows distinct changes in the Raman spectra of the matrices. Weak influences of the impurity on site symmetry and on the vibron–phonon and phonon–phonon coupling mechanisms of the anisotropic matrices N2 and O2 are obtained. An additional line, observed in the vibron–phonon structure of N2 and O2, is interpreted as a coupled resonant mode. Direct evidence of a resonant mode at 39 cm−1 is only obtained in the system containing 1% D2 in N2.

Raman studies on N2 – rare-gas mixed crystals

Raman spectra of solid nitrogen and of mixed N2–rare gases (RG-Ar, Kr, Xe) are investigated over a concentration range as wide as allowed by the pertinent experimental conditions. Both the v(N2) and the lattice-mode region are studied. The effects of changes in RG impurity concentrations and temperature variations are described. Results are discussed in terms of "matrix effects," like frequency shifts, on one hand and "solid-state" effects, like resonance splitting of the vibrational transition, vibron–phonon coupling, and one- and two-phonon transitions, on the other.

Quasidiffusive transport of a broadband phonon distribution

The characteristics of the quasidiffusive transport of an initially broadband phonon distribution are obtained experimentally by vibronic sideband phonon spectroscopy and compared to the characteristics predicted theoretically. It is shown that the transport, which is governed by the simultaneous presence of elastic and anharmonic phonon scattering, has a number of distinctive features which differ to some extent from those observed for an initially monochromatic phonon distribution.

Influence of temperature on elementary excitations in solid oxygen by Raman studies

Detailed Raman investigations on gaseous, liquid, and solid oxygen in its α, β, and γ phases were made. Some new features observed in the spectra are a temperature dependent splitting of the vibrational line in γ-O2 and a vibron–phonon combination in α- and β-O2. In the low energy range we measured the librational modes; one of them shows a characteristic soft mode behavior near the α–β phase transition. The influence of temperature on the librational linewidth is explained in terms of the anharmonic interaction. These data give new information about the assignment of some of the modes.

Polarized infrared absorption spectrum of Cs2UO2Cl4

Polarized infrared spectra have been measured on single crystals of Cs2UO2Cl4 at room and liquid-nitrogen temperatures. All features between 2000 and 400 cm–1 are assigned to (a) fundamental modes of 16OU16OCl2–4, 17OU16OCl2–4 and 18OU16OCl2–4 ions, (b) combination bands involving two internal anion modes and (c) vibron–phonon modes involving one internal mode and one lattice mode. The structure of the features (c) reflects the dispersion of the lattice modes. These data, together with new Raman spectra, enable assignments of all the internal modes of the UO2Cl2–4 ion to be proposed.

A p r i o r i rates of phonon-assisted v–v transfers among CO and N2 molecules in N2 matrices

The method of computing matrix shifts, splittings, and sideband intensities of the vibrational fundamental bands in solid N2 and CO has been extended to the calculation of resonant and off-resonant v–v transfer rates. The contribution of mechanical anharmonicity, i.e., vibron–phonon coupling through third- and higher-order terms in the potential, is found to be negligible for phonon-assisted transfer rates, similar to the situation in ir and Raman sideband intensities. Electrical anharmonicity, i.e., the modulation of vibron resonance energies by phonon-type motions, produces off-resonant v–v rates which are larger than those due to mechanical anharmonicity by two orders of magnitude. Analytical expressions have been derived for the quantitative determination of rates based on static multipole–multipole interactions, and contributions by repulsive exchange forces have been calculated numerically. The calculated rates are consistent with observations in a system of CO diluted in N2 and they explain the observed temperature behavior of endothermic v–v processes in pure solid N2 where two and more phonons are involved. It is found that molecular librations dominate over translational motions in all of these phonon-assisted rates, even in the repulsive contributions.

Calculation of the vibron band and of the vibron–phonon combination bands in the ir and Raman spectra of solid α-CO

The observed gas-to-solid shift, splitting, and intensity ratio of the vibrational band in solid α-CO and the shifts of CO isotopes suspended in solid 12C16O and in N2 matrices have been studied by calculations based on known gas-phase and matrix properties. Equations and numerical results for the crystal sums of the various intermolecular interactions are tabulated. It has been shown that the random distribution of the two possible orientations of the CO molecules modifies the dipole–dipole interaction in such a way that the ir spectrum is not affected whereas the Raman spectrum is spread over the entire vibron band. The splitting in the ir spectrum reported by Dubost is interpreted as LO–TO splitting which becomes visible because of nonnormal incidence. The vibron–phonon combination bands have been calculated using electrical anharmonicities, i.e., the bilinear dependence of the polarizability and of the dipole moment on internal and external coordinates. An ellipsoidal cavity has been assumed to compute the influence of the local field on the Raman intensity of the split vibron bands and on the intensity ratio of the vibron and the sideband in solid α-CO and α-N2.

Theory of vibrational relaxation and infrared absorption in condensed media

In this paper, we develop a theoretical model for vibrational relaxation of vibrons in condensed media based on the breakdown of the adiabatic separation of vibron (high frequency) and phonon (low frequency) motion. The conditions under which the adiabatic approximation of the vibron–phonon system is valid are examined. The present model is also applied to the ir absorption in condensed media; the band shape function and the dipole correlation function are obtained. The relation between the vibrational relaxation and the ir absorption is discussed.