Spectrum Of Vibrational Modes Research Articles

Optimal Experimental Condition of IR pMAIRS Calibrated by Using an Optically Isotropic Thin Film Exhibiting the Berreman Effect.

Infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) is a useful spectroscopic tool for revealing the molecular anisotropic structure in a thin film, which is used for the molecular orientation analysis of many functionalized organic thin films. Infrared pMAIRS provides both in-plane (IP) and out-of-plane (OP) vibrational mode spectra, which are influenced by the choice of the angles of incidence, i.e., angle set. To obtain quantitatively reliable pMAIRS spectra, therefore, the optimal angle set must be revealed. In a former study, an optimization study was carried out on a silicon substrate by using the band intensity ratio of the IP and OP spectra of highly oriented molecules in a thin film, which has a problem that the optimized results cannot be used for another substrate. In the present study, a totally new idea using an optically isotropic thin film as a standard sample is proposed to comprehensively explore the optimal angle set on various substrates: the band shift due to the Berreman effect of a strongly absorbing compound is used, instead of the band intensity. This new approach makes the pMAIRS calibration for various substrates a much easier task. With the optimal angle set, the molecular orientation angle in the film calculated by the pMAIRS spectra is also found to be reliable quantitatively. This technique opens a user-friendly way to a reliable molecular orientation analysis in an ultrathin film using IR pMAIRS.

Observation of Fano line shapes in infrared vibrational spectra of CO2 adsorbed on Cu(997) and Cu(111)

Adsorption states of carbon dioxide on the Cu(997) and Cu(111) surfaces were investigated by infrared reflection absorption spectroscopy, temperature programmed desorption, and X-ray photoelectron spectroscopy. CO2 molecules are physisorbed on the Cu(997) surface at temperatures below 70 K; neither chemisorption nor dissociation of CO2 occurs on the Cu(997) surface at this low temperature. However, the vibrational spectra of adsorbed CO2 depend significantly on the substrate temperature and coverage. IR spectra of CO2 vibrational modes at 70 K show asymmetric Fano line shapes, while only normal absorption bands are observed when CO2 is adsorbed at 20 K. Fano line shapes are also observed for CO2 on Cu(111) at 85 K. The observation of Fano effect indicates the coupling between the electronic continuum states of the Cu surface and the internal vibrational modes of CO2 even in such physisorbed system.

Tailored Hypersound Generation in Single Plasmonic Nanoantennas.

Ultrashort laser pulses impinging on a plasmonic nanostructure trigger a highly dynamic scenario in the interplay of electronic relaxation with lattice vibrations, which can be experimentally probed via the generation of coherent phonons. In this Letter, we present studies of hypersound generation in the range of a few to tens of gigahertz on single gold plasmonic nanoantennas, which have additionally been subjected to predesigned mechanical constraints via silica bridges. Using these hybrid gold/silica nanoantennas, we demonstrate experimentally and via numerical simulations how mechanical constraints allow control over their vibrational mode spectrum. Degenerate pump-probe techniques with double modulation are performed in order to detect the small changes produced in the probe transmission by the mechanical oscillations of these single nanoantennas.

Far infrared spectrum of Methanol-D2 in the lowest torsional state (e0)

The infrared (IR) and far infrared (FIR) absorption spectra have been measured for the Methanol-D1 and D2 species in the wave number range of 20–1200cm−1 at the Justus Liebig Universität in Giessen, Germany using a Brüker Fourier Transform spectrometer at a resolution of 0.002cm−1. The spectra looked very complicated but analyses were possible for the lower lying states. At higher wave number regions spectra of few vibrational fundamental modes were visible. In this communication, the details of the assignments and analyses of the lowest lying internal rotational state (e0) for Methanol-D2 are reported. A catalog of about 900 assigned spectral lines has been prepared and is made available from the author to conserve space. As application of this work, it was possible to assign some optically pumped FIR laser lines to quantum states. The results should be useful for “radio astronomers” and in the field of optically pumped FIR and Sub-millimeter Wave (SMMW) lasers which are used in Tokamaks for plasma diagnostics and as local oscillators in radio detection from space.

Prediction of novel alloy phases of Al with Sc or Ta.

Using the evolutionary optimization algorithm, as implemented in the USPEX crystal predictor program, and first principles total energy calculations, the compositional phase diagrams for Al-Sc and Al-Ta alloy systems at zero temperature and pressure have been calculated. In addition to the known binary intermetallic phases, new potentially stable alloys, AlSc3 and AlTa7, have been identified in the Al-poor region of the phase diagram. The dynamic and thermal stability of their lattices has been confirmed from the calculated vibrational normal mode spectra in the harmonic approximation.

Construction of a thermodynamic potential for the water ices VII and X

We employ a combination of density functional theory (DFT), molecular dynamics (MD), and a variety of advanced postprocessing methods to construct an analytic thermodynamic potential (free energy) for ices VII and X. In particular, the temperature-dependent part of the free energy function is constructed using entropy data obtained via the spectrum of vibrational modes from the MD simulations. Conceptional challenges due to the partial absence of stable zero-temperature states and proton disorder are overcome by performing calculations of representative crystalline states combined with a three-stage fitting procedure of data from MD simulations and static DFT calculations. The influence of the exchange-correlation functional is extensively discussed, and a comparison with available experiments is made as well and generally shows good agreement. This work is of significant importance for astrophysical applications, such as the interior modeling of dense icy planets and moons.

Long-Range Protein Vibrations Dependence on Ligand Binding: Rate Promoting Motions

It has been suggested that long-range intramolecular vibrations in enzymes may provide efficient access to intermediate state configurations, enhancing catalytic turnover rates. Recently we have successfully measured these long-range protein vibrations using an anisotropic THz near-field technique to measure protein crystals, called crystal anisotropy terahertz microscopy (CATM). The method isolates these motions from the isotropic librational background, revealing narrow band resonances. Recent measurements on free chicken-egg white lysozyme (CEWL) and tri-N-acetylglucosamine (3NAG) inhibitor bound CEWL reveal dramatic and reproducible changes in the intramolecular vibrational mode spectra with binding. A large resonance at ∼70 cm−1 for free CEWL, is somewhat suppressed with binding, whereas a new resonance is observed at ∼ 40 cm−1, with binding. This dramatic change in the spectra with binding confirms that the observed resonances arise from intramolecular vibrations, and not crystal phonons. Using normal mode analysis, our calculated CATM spectra find a consistent increase in the low frequency signal with binding. While the density of states does not change appreciably with binding, the overall direction of motion shifts for vibrations in the frequency range where there is a large change in the optical signal. The change in the direction of the vibrational motion results in a change in the motion of the charge distribution and therefore, the dipole derivative, leading to the sensitivity to the trajectories in the terahertz optical absorbance. The remaining question is if these specific motions participate in the structural preorganization of the binding site, and thus promote catalytic activity. We will discuss measurements to answer this long standing question. This work was supported by NSF MRI∧2 grant DBI295998.

Hydrogen-Induced Reconstruction of Cu(100): Two-Dimensional and One-Dimensional Structures of Surface Hydride

Adsorption of atomic hydrogen has a remarkable influence on the structure and morphology of the Cu(100) surface. We have examined two specific situations; the well-known reconstructed p(2 × 2)-H surface and a one-dimensional (6 × 1)-H surface resembling the stripe reconstruction observed in an electrochemical environment. High resolution electron energy loss spectroscopy and density functional calculations show that the H atoms occupy only surface sites and that H bonding in induced 3-fold Cu surface hollow sites is a structural key element. The calculations support a transient scenario in which H absorption in subsurface sites is a critical intermediate step in the reconstruction of the Cu surface. We find clear evidence in terms of characteristic properties, like the H–Cu bond lengths and the dipole excited vibrational mode spectra, that these structures consist of two-dimensional and one-dimensional structures of surface hydride.

Laboratory spectra of CO2 vibrational modes in planetary ice analogs

Laboratory spectra have shown that CO2 is a powerful diagnostic tool for analyzing infrared data from remote observations, as it has been detected on icy moons in the outer Solar System as well as dust grain surfaces in the interstellar medium (ISM). IR absorption band profiles of CO2 within ice mixtures containing H2O and CH3OH change with respect to temperature and mixture ratios. In this particular study, the ν3 CO2 asymmetric stretch mode near 4.3μm (2350cm−1), overtone mode near 1.97μm (5080cm−1), and the combination bands near 2.7μm (3700cm−1), 2.8μm (3600cm−1), and 2.02μm (4960cm−1), are systematically observed in different mixtures with H2O and CH3OH in temperature ranges from 15K to 150K. Additionally, some high-temperature deposits (T>50K) of H2O, CH3OH, and CO2 ice mixtures were performed and it was discovered that CO2 may deposit out at higher temperatures than previously recorded. These data may then be used to interpret infrared observational data obtained from icy surfaces in the outer Solar System and beyond.

Raman spectra of vibrational and librational modes in methane clathrate hydrates using density functional theory

The sI type methane clathrate hydrate lattice is formed during the process of nucleation where methane gas molecules are encapsulated in the form of dodecahedron (5(12)CH(4)) and tetrakaidecahedron (5(12)6(2)CH(4)) water cages. The characterization of change in the vibrational modes which occur on the encapsulation of CH(4) in these cages plays a key role in understanding the formation of these cages and subsequent growth to form the hydrate lattice. In this present work, we have chosen the density functional theory (DFT) using the dispersion corrected B97-D functional to characterize the Raman frequency vibrational modes of CH(4) and surrounding water molecules in these cages. The symmetric and asymmetric C-H stretch in the 5(12)CH(4) cage is found to shift to higher frequency due to dispersion interaction of the encapsulated CH(4) molecule with the water molecules of the cages. However, the symmetric and asymmetric O-H stretch of water molecules in 5(12)CH(4) and 5(12)6(2)CH(4) cages are shifted towards lower frequency due to hydrogen bonding, and interactions with the encapsulated CH(4) molecules. The CH(4) bending modes in the 5(12)CH(4) and 5(12)6(2)CH(4) cages are blueshifted, though the magnitude of the shifts is lower compared to modes in the high frequency region which suggests bending modes are less affected on encapsulation of CH(4). The low frequency librational modes which are collective motion of the water molecules and CH(4) in these cages show a broad range of frequencies which suggests that these modes largely contribute to the formation of the hydrate lattice.

Vibrationally Assisted Polariton‐Relaxation Processes in Strongly Coupled Organic‐Semiconductor Microcavities

AbstractIf a semiconductor with an electronic transition that approximates a two‐level system is placed within an optical cavity, strong coupling can occur between the confined photons and the semiconductor excitons. This coupling can result in the formation of cavity polariton states that are a coherent superposition of light and matter. If the material in the cavity is an organic semiconductor, it has been predicted that interactions between Frenkel excitons, polaritons, and molecular vibrational modes will have a profound role in defining the overall relaxation dynamics of the system. Here, using temperature‐dependent spectroscopy on a microcavity containing a J‐ aggregated cyanine dye, it is shown that a spectrum of localized vibrational modes (identified by Raman scattering) enhances the population of certain polaritonic modes by acting as an energy‐loss channel to the excitons as they undergo scattering. Our work demonstrates that simultaneous control of the optical properties of a cavity and the vibrational structure of a molecular dye could promote the efficient population of k = 0 polariton states, from which lasing and other cooperative phenomena may occur.

A Polarizable Water Model for Intramolecular and Intermolecular Vibrational Spectroscopies

We have developed a polarizable water model for classical molecular dynamics simulations of vibrational spectroscopies, which covers from low-frequency intermolecular modes to high-frequency intramolecular vibrational modes. The model utilizes the ab initio derived geometry-dependent multipole moment surfaces to depict the instantaneous charge density of a water molecule. Multipoles up to quadrupole are included for the permanent multipoles, while those up to dipole are included for the induced multipoles. The polarization of molecules is described by a distributed polarizability model. At room temperature, the present model is able to reproduce experimental infrared and Raman spectra of intramolecular vibrational modes, except for the blue peak shift due to a limitation of the classical simulation based on a quantum mechanical potential. The calculated infrared spectrum for low-frequency intermolecular modes agreed reasonably well with the experimental signals.

Vibrational mode and dielectric function spectra of BGaP probed by Raman scattering and spectroscopic ellipsometry

The influence of boron incorporation in BxGa1−xP (0 < x ≤ 6%) layers on the dielectric function spectrum and on the vibrational mode spectrum has been studied. BGaP layers were grown by metal-organic vapor phase epitaxy, using a GaP interlayer, on Si and, for reference purposes, also on GaP substrates. The boron content of the layers was determined by high-resolution x-ray diffraction. Two vibrational modes arising from the two boron isotopes 10BGa and 11BGa were observed in the Raman spectrum, increasing in scattering strength and shifting to higher frequencies with increasing boron content at a rate of (1.40 ± 0.20) cm−1/% and (1.41 ± 0.14) cm−1/% for the 10BGa and 11BGa modes, respectively. Spectroscopic ellipsometry was used to asses the pseudodielectric function of the BxGa1−xP layers. The main effect of boron incorporation is a strong broadening of the E1 interband transition with increasing boron content due to alloy disorder and inhomogeneous random strains introduced by the large size mismatch of boron. However, upon the addition of arsenic to BGaP, i.e., forming BxGa1−xAsyP1−y with y ≈ 10%, a clear low-energy shift of the E1 interband transition is resolved. A small low-energy shift (≈ 20 meV) of the lowest direct bandgap has also been observed for BxGa1−xP layers with x ≤ 0.7% grown on GaP substrates.

Vibrational signature of the Si–N defect in Si-doped GaN xAs 1− x

Using first principles density functional theory in a supercell approach, we calculate the full spectrum of localized vibration modes associated with the Si–N defect complex in GaAs. Two recently proposed structures are investigated in detail: (a) substitutional Si on a Ga site with substitutional N on an adjacent As site [ (Si Ga–N As) q , where q is the charge state of the defect complex], and (b) Si and N atoms forming a split-interstitial defect on an As site [ (Si–N) q As]. All the localized mode frequencies for both defect structures are calculated in the relevant charge states. We have also calculated the formation energy of the (Si Ga–N As) q defect as a function of Si–N separation, finding that the configuration with the Si on a second-nearest-neighbor Ga site is more favorable in the positive charge state than the configuration with the Si on a nearest-neighbor Ga site to the N atom.

Phononic Band Gaps in One-dimensional Phononic Crystals with Nanoscale Periodic Corrugations at Interfaces. FDTD and PWM Simulations

We present and apply two complementary calculation methods used in phononic crystal studies: the finite difference time domain (FDTD) method and the plain wave method (PWM). The FDTD technique allows to simulate the time dependence of a wave packet of vibrational modes propagating through a composite and to determine the transmission coefficient. The PWM method is used for the determination of the phononic dispersion relation in systems with discrete translational symmetry. We use both methods for investigating the effect of periodic interface perturbations on the spectrum of longitudinal vibrational modes in 1D phononic crystals composed of semiconducting materials. The material parameters in the composites under consideration are modulated in the nanoscale.

Analysis of experimental positron-molecule binding energies

Experiments show that positron annihilation on molecules frequently occurs via capture into vibrational Feshbach resonances. In these cases, the downshifts in the annihilation spectra from the vibrational mode spectra provide measures of the positron-molecule binding energies. An analysis of these binding energy data is presented in terms of the molecular dipole polarizability, the permanent dipole moment, and the number of π bonds in aromatic molecules. The results of this analysis are in reasonably good agreement with other information about positron-molecule bound states. Predictions for other targets and promising candidate molecules for further investigation are discussed.

Jammed frictionless disks: Connecting local and global response

By calculating the linear response of packings of soft frictionless disks to quasistatic external perturbations, we investigate the critical scaling behavior of their elastic properties and nonaffine deformations as a function of the distance to jamming. Averaged over an ensemble of similar packings, these systems are well described by elasticity, while in single packings we determine a diverging length scale l* up to which the response of the system is dominated by the local packing disorder. This length scale, which we observe directly, diverges as 1/Deltaz , where Deltaz is the difference between contact number and its isostatic value, and appears to scale identically to the length scale which had been introduced earlier in the interpretation of the spectrum of vibrational modes. It governs the crossover from isostatic behavior at the small scale to continuum behavior at the large scale; indeed we identify this length scale with the coarse graining length needed to obtain a smooth stress field. We characterize the nonaffine displacements of the particles using the displacement angle distribution, a local measure for the amount of relative sliding, and analyze the connection between local relative displacements and the elastic moduli.

Analysis of the vibrational mode spectrum of a linear chain with spatially exponential properties

We deduce the dynamic frequency-domain-lattice Green’s function of a linear chain with properties (masses and next-neighbor spring constants) of exponential spatial dependence. We analyze the system as discrete chain as well as the continuous limiting case which represents an elastic 1D exponentially graded material. The discrete model yields closed form expressions for the N × N Green’s function for an arbitrary number N = 2 , … , ∞ of particles of the chain. Utilizing this Green’s function yields an explicit expression for the vibrational mode density. Despite of its simplicity the model reflects some characteristics of the dynamics of a 1D exponentially graded elastic material. As a special case the well-known expressions for the Green’s function and oscillator density of the homogeneous linear chain are contained in the model. The width of the frequency band is determined by the grading parameter which characterizes the exponential spatial dependence of the properties. In the limiting case of large grading parameter, the frequency band is localized around a single finite frequency where the band width tends to zero inversely with the grading parameter. In the continuum limit the discrete Green’s function recovers the Green’s function of the continuous equation of motion which takes in the time domain the form of a Klein–Gordon equation.

Vibrational modes of metal nanoshells and bimetallic core-shell nanoparticles

We theoretically study the spectrum of radial vibrational modes in composite metal nanostructures such as bimetallic core-shell particles and metal nanoshells with dielectric core in an environment. We calculate frequencies and damping rates of fundamental (breathing) modes for these nanostructures along with those of two higher-order modes. For metal nanoshells, we find that the breathing mode frequency is always lower than the one for solid particles of the same size, while the damping is higher and increases with a reduction in the shell thickness. We identify two regimes that can be characterized as weakly damped and overdamped vibrations in the presence of external medium. For bimetallic particles, we find periodic dependence of frequency and damping rate on the shell thickness with period being determined by the mode number. For both types of nanostructures, the frequency of higher modes is nearly independent of the environment, while the damping rate shows a strong sensitivity to the outside medium.

Local vibrations in disordered solids studied via single-molecule spectroscopy: Comparison with neutron, nuclear, Raman scattering, and photon echo data

The energy spectrum of low-frequency vibrational modes (LFMs) in three disordered organic solids---amorphous polyisobutylene (PIB), toluene and deuterated toluene glasses, weakly doped with fluorescent chromophore molecules of tetra-tert-butylterrylene (TBT) has been measured via single-molecule (SM) spectroscopy. Analysis of the individual temperature dependences of linewidths of single TBT molecules allowed us to determine the values of the vibrational mode frequencies and the SM-LFM coupling constants for vibrations in the local environment of the molecules. The measured LFM spectra were compared with the ``Boson peak'' as measured in pure PIB by inelastic neutron scattering, in pure toluene glass by low-frequency Raman scattering, in doped toluene glass by nuclear inelastic scattering, and with photon echo data. The comparative analysis revealed close agreement between the spectra of the local vibrations as measured in the present study and the literature data of the Boson peak in PIB and toluene. The analysis has also the important result that weak doping of the disordered matrices with nonpolar probe molecules whose chemical composition is similar to that of the matrix molecules does not influence the observed vibrational dynamics markedly. The experimental data displaying temporal stability on the time scale of a few hours of vibrational excitation parameters in local surroundings was obtained for the first time both for polymer and molecular glass.