Collective Vibrational Modes Research Articles

Angular momentum generation in nuclear fission.

When a heavy atomic nucleus splits (fission), the resulting fragments are observed to emerge spinning1; this phenomenon has been a mystery in nuclear physics for over 40 years2,3. The internal generation of typicallysix or seven units of angular momentum in each fragment is particularly puzzling for systems that start with zero, or almost zero, spin. There are currently no experimental observations that enable decisive discrimination between the many competing theories for the mechanism that generates the angular momentum4-12. Nevertheless, the consensus is that excitation of collective vibrational modes generates the intrinsic spin before the nucleus splits (pre-scission). Here we show that there is no significant correlation between the spins of the fragment partners, which leads us to conclude that angular momentum in fission is actually generated after the nucleus splits (post-scission). We present comprehensive data showing that the average spin is strongly mass-dependent, varying in saw-tooth distributions. We observe no notable dependence of fragment spin on the mass or charge of the partner nucleus, confirming the uncorrelated post-scission nature of the spin mechanism. To explain these observations, we propose that the collective motion of nucleons in the ruptured neck of the fissioning system generates two independent torques, analogous to the snapping of an elastic band. A parameterization based on occupation of angular momentum states according to statistical theory describes the full range of experimental data well. This insight into the role of spin in nuclear fission is not only important for the fundamental understanding and theoretical description of fission, but also has consequences for the γ-ray heating problem in nuclear reactors13,14, for the study of the structure of neutron-rich isotopes15,16, and for the synthesis and stability of super-heavy elements17,18.

Study of the displacement transition in SF6 + Kr binary films on graphite using infrared spectroscopy and ellipsometry

The hysteretic displacement of an SF6 monolayer on graphite by Kr was studied using infrared reflection absorption spectroscopy supplemented by ellipsometry. The SF6 molecule has a strong IR-active vibrational mode (ν3) at 948.0 cm−1. The coverage and local density of the SF6 monolayer are inferred from the ν3 absorption peak area and the blue shift of the ν3 mode, respectively, the latter due to dynamic dipole coupling between SF6 molecules in the layer. Throughout the temperature range 65.3 to 110.2 K, a first order transition with large hysteresis was found in the SF6 monolayer displacement-recovery cycle driven by Kr spreading pressure. The hysteresis is asymmetric, with displacement going to completion at a sharp pressure, while the initial ~10% of recovery occurs much closer to the displacement pressure than the pressure allowing full recovery, especially above 95 K. Over the temperature range studied, an upper limit of 0.25% on the solubility of Kr in the SF6 layer was determined by the absence of a significant change in the collective vibrational mode frequency preceding displacement. Above 92 K, spectra indicate a small solubility of SF6 in the Kr monolayer, up to 4% approaching displacement of the Kr layer at T ≥ 98 K. Below 74 K the kinetics of SF6 displacement to bulk is sufficiently slow that the monolayer is readily compressed to a metastable bilayer covering half the surface, with both layers apparently 2 × 2 commensurate with the graphite substrate. The decay time constant of the confined bilayer to bulk SF6 crystallites is about 4 h at 65.3 K.

Vibrational modes optimization and terahertz time-domain spectroscopy of L-Lysine and L-Lysine hydrate

To date, it is challengeable to clearly identify and accurately characterize molecular vibrational modes of an amino acid and its hydrate. In this work, the terahertz absorption spectra of L-Lysine (Lys) and L-Lysine hydrate (Lys·H2O) have been measured by terahertz time-domain spectroscopy (THz-TDS) at the range of 0.3–2.5 THz under room temperature. The structural changes of Lys and Lys·H2O are sensitively observed by THz spectroscopy. Based on density functional theory (DFT), the vibrational frequencies of Lys and Lys·H2O are analyzed, the Lys single molecule model and the most optimal Lys·H2O single molecule model are established. The collective vibrational modes of Lys and Lys·H2O have been identified by potential energy distribution (PED) analysis and molecular vibrational animation of GaussView. The results demonstrate that the collective vibrational modes of Lys and Lys·H2O at various frequencies are originated from the dihedral torsion or the bond angle bend of different molecular chains in a single molecule, which drives the vibrations of related groups. Therefore, the combination of THz-TDS technology and PED analysis opens up an opportunity for identifying Lys and its hydrate, which provides a theoretical reference for studying vibrational modes of biomolecules.

Correlation and spectrum of dust acoustic waves in a radio-frequency plasma using PK-4 on the International Space Station

Dust acoustic waves were investigated in an experiment under microgravity conditions, using the European Space Agency-Roscosmos facility PK-4 on the International Space Station. A three-dimensional dust cloud was confined in a glow-discharge plasma powered by a radio-frequency coil in low-pressure neon gas. Low-frequency dust acoustic waves (DAWs) were spontaneously excited due to the flowing ions in the plasma. Dust motion was imaged using video cameras, yielding the position for individual dust particles. Besides these particle-level measurements, the dust was also measured as a fluid continuum to obtain its velocity and density fluctuations. A space–time diagram of the dust-fluid velocity reveals that the waves changed properties as they propagated across the dust cloud, in the direction of the ion flow. Density fluctuations were characterized by a wave spectrum, indicating the emergence of collective vibration modes in a broad range of wave numbers. The decay of the modes was characterized by density correlation functions. Fitting the correlation functions to an analytical model yields an experimental measurement of the dispersion relation for DAWs, which is compared to that of linear theory.

Charge-exchange dipole excitations in deformed nuclei

Background: The electric giant-dipole resonance (GDR) is the most established collective vibrational mode of excitation. A charge-exchange analog, however, has been poorly studied in comparison with the spin (magnetic) dipole resonance (SDR).Purpose: I investigate the role of deformation on the charge-exchange dipole excitations and explore the generic features as an isovector mode of excitation.Methods: The nuclear energy-density functional method is employed for calculating the response functions based on the Skyrme-Kohn-Sham-Bogoliubov method and the proton-neutron quasiparticle-random-phase approximation.Results: The deformation splitting into $K=0$ and $K=\ifmmode\pm\else\textpm\fi{}1$ components occurs in the charge-changing channels and is proportional to the magnitude of deformation as is well known for the GDR. For the SDR, however, a simple assertion based on geometry of a nucleus cannot be applied for explaining the vibrational frequencies of each $K$ component. A qualitative argument on the strength distributions for each component is given based on the non-energy-weighted sum rules taking nuclear deformation into account. The concentration of the electric dipole strengths in low energy and below the giant resonance is found in neutron-rich unstable nuclei.Conclusions: The deformation splitting occurs generically for the charge-exchange dipole excitations as in the neutral channel. The analog pygmy dipole resonance can emerge in deformed neutron-rich nuclei as well as in spherical systems.

Investigation of the Correlations between Amino Acids, Amino Acid Mixtures and Dipeptides by Terahertz Spectroscopy

Terahertz (THz) spectroscopy is regarded as a professional technology to probe collective molecular vibrational modes and intensity of bio–molecules, which is potential to analyze amino acids and short peptides. In this study, THz spectroscopy is utilized to investigate the correlations of glycine (Gly), Serine (Ser), amino acid mixtures of Gly/Ser, and dipeptides (GS and SG) in the frequency range of 0.3–2.5 THz. The results manifest that the collective molecular vibrational intensity of Ser is larger than Gly, implying that the hydroxyl methyl group (–CH2–OH) on the side chain of Ser exhibits larger motions than that of –H in Gly. Additionally, the THz spectra of amino acid mixtures of Gly/Ser fit the linear addition of the individual amino acid absorption spectrum. Herein, the THz spectra of dipeptide GS and its isomer SG are distinguished according to the different collective molecular vibrations produced by the terminal groups on the carbon skeleton. Compared to physical amino acid mixtures of Gly/Ser, the dipeptides exhibit diverse THz spectra, indicating that the different collective vibrational modes are caused by peptide bonds and terminal groups. These solid evidences confirm that THz spectroscopy can be availably used to analyze the correlations of amino acids, amino acid mixtures, and dipeptides, as well as to identify dipeptide and its isomer from molecular level.

Collective excitations in α-helical protein structures interacting with the water environment

ABSTRACT Low-frequency vibrational excitations of protein macromolecules in the terahertz frequency region are suggested to contribute to many biological processes such as enzymatic catalysis, intra-protein energy/charge transport, recognition, and allostery. To explain high effectiveness of these processes, two possible mechanisms of the long-lived excitation were proposed by H. Fröhlich and A.S. Davydov, which relate to either vibrational modes or solitary waves, respectively. In this paper, we developed a quantum dynamic model of vibrational excitation in α-helical proteins interacting with the aqueous environment. In the model, we distinguished three coupled subsystems, i.e., (i) a chain of hydrogen-bonded peptide groups (PGs), interacting with (ii) the subsystem of the side-chain residuals which in turn interact with (iii) the environment, surrounding water responsible for dissipation and fluctuation in the system. It was shown that the equation of motion for phonon variables of the PG chain can be transformed to nonlinear Schrodinger equation which admits bifurcation into the solution corresponding to the weak-damped vibrational modes (Fröhlich-type regime) and Davydov solitons. A bifurcation parameter is derived through the strength of phonon–phonon interaction between the side-chains and hydration-shell water molecules. As shown, the energy of these excited states is pumped through the interaction of the side-chains with fluctuating water environment of the proteins. The suggested mechanism of the collective vibrational mode excitation is discussed in connection with the recent experiments on the long-lived collective protein excitations in the terahertz frequency region and vibrational energy transport pathways in proteins.

Light stability of the isolated CP43 studied by infrared and terahertz spectroscopy

Here, the light-induced changes of the core antenna complex chlorophyll protein 43 (CP43) isolated from PSII of spinach were assessed using infrared and terahertz (THz) spectroscopy. We found that the collective vibrational modes at low frequencies and the secondary structures of CP43 were altered upon illumination. The helix changed more than any other secondary structure. CP43 is more easily damaged by illumination than PSII. Infrared and THz spectroscopy have different functions in protein studies. THz spectroscopy has potential in evaluating the stability of proteins.

Spectroelectrochemical and Density Functional Theory Study of Squaric Acid Adsorption and Oxidation at Gold Thin Film and Single Crystal Electrodes

The adsorption and oxidation of squaric acid (H2C4O4, H2SQ) at gold single crystal and thin-film electrodes with preferential (111) orientation were studied spectroelectrochemically in perchloric acid solutions. The existence of reversible adsorption-desorption processes in the double-layer region is reflected by structure-sensitive voltammetric features. Infrared reflection absorption spectroscopy experiments carried out with Au(111) and Au(100) single-crystal surfaces in 10 mM H2SQ solutions show potential-dependent adsorbate bands at ca. 1780-1785 and 1511-1577 cm-1 for potentials below 1.00 V RHE. The increasing sensitivity of the attenuated total reflection (ATR)-surface-enhanced infrared reflection absorption (SEIRA) experiments allows the detection of similar features for much lower H2SQ concentrations. According to density functional theory (DFT) calculations, these bands can be assigned to adsorbed squarate anions which are bonded to the gold surfaces in a bidentate configuration through two oxygen atoms, with the molecular plane perpendicular to the metal surface. For 10 mM H2SQ solutions, additional bands are detected in the ATR-SEIRA spectra at ca. 1630 cm-1 both in water and deuterium oxide solutions. Even if this frequency fits with one of the vibrational modes of adsorbed bisquarate, DFT calculations provide an alternative explanation for this potential-dependent feature that could be ascribed to collective vibrational modes of adsorbed squarate appearing at high adsorbate coverage. The existence of in-phase and out-of-phase contributions under these conditions would also explain the broadening and/or splitting of the observed bands. DFT calculations also show that squaric acid molecules adsorb very weakly at the gold surfaces and can be discarded as the origin of the observed infrared bands. The external reflection infrared spectra obtained for gold single-crystal electrodes in the H2SQ oxidation region show bands for dissolved carbon dioxide molecules as the main product. Bands for adsorbed bicarbonate anions formed from carbon dioxide are detected in the ATR-SEIRA spectra.

Two and three electrons on a sphere: A generalized Thomson problem

Generalizing the classical Thomson problem to the quantum regime provides an ideal model to explore the underlying physics regarding electron correlations. In this work, we systematically investigate the combined effects of the geometry of the substrate and the symmetry of the wave function on correlations of geometrically confined electrons. By the numerical configuration interaction method in combination with analytical theory, we construct symmetrized ground-state wave functions; analyze the energetics, correlations, and collective vibration modes of the electrons; and illustrate the routine for the strongly correlated, highly localized electron states with the expansion of the sphere. This work furthers our understanding about electron correlations on confined geometries and shows the promising potential of exploiting confinement geometry to control electron states.

High-Precision Megahertz-to-Terahertz Dielectric Spectroscopy of Protein Collective Motions and Hydration Dynamics.

The low-frequency collective vibrational modes in proteins as well as the protein-water interface have been suggested as dominant factors controlling the efficiency of biochemical reactions and biological energy transport. It is thus crucial to uncover the mystery of the hydration structure and dynamics as well as their coupling to collective motions of proteins in aqueous solutions. Here, we report dielectric properties of aqueous bovine serum albumin protein solutions as a model system using an extremely sensitive dielectric spectrometer with frequencies spanning from megahertz to terahertz. The dielectric relaxation spectra reveal several polarization mechanisms at the molecular level with different time constants and dielectric strengths, reflecting the complexity of protein-water interactions. Combining the effective-medium approximation and molecular dynamics simulations, we have determined collective vibrational modes at terahertz frequencies and the number of water molecules in the tightly bound and loosely bound hydration layers. High-precision measurements of the number of hydration water molecules indicate that the dynamical influence of proteins extends beyond the first solvation layer, to around 7 Å distance from the protein surface, with the largest slowdown arising from water molecules directly hydrogen-bonded to the protein. Our results reveal critical information of protein dynamics and protein-water interfaces, which determine biochemical functions and reactivity of proteins.

Subwavelength hyperspectral THz studies of articular cartilage

Terahertz-spectroscopy probes dynamics and spectral response of collective vibrational modes in condensed phase, which can yield insight into composition and topology. However, due to the long wavelengths employed (λ = 300 μm at 1THz), diffraction limited imaging is typically restricted to spatial resolutions around a millimeter. Here, we demonstrate a new form of subwavelength hyperspectral, polarization-resolved THz imaging which employs an optical pattern projected onto a 6 μm-thin silicon wafer to achieve near-field modulation of a co-incident THz pulse. By placing near-field scatterers, one can measure the interaction of object with the evanescent THz fields. Further, by measuring the temporal evolution of the THz field a sample’s permittivity can be extracted with 65 μm spatial resolution due to the presence of evanescent fields. Here, we present the first application of this new approach to articular cartilage. We show that the THz permittivity in this material varies progressively from the superficial zone to the deep layer, and that this correlates with a change in orientation of the collagen fibrils that compose the extracellular matrix (ECM) of the tissue. Our approach enables direct interrogation of the sample’s biophysical properties, in this case concerning the structure and permittivity of collagen fibrils and their anisotropic organisation in connective tissue.

Activity-induced instability of phonons in 1D microfluidic crystals.

One-dimensional crystals of passively-driven particles in microfluidic channels exhibit collective vibrational modes reminiscent of acoustic 'phonons'. These phonons are induced by the long-range hydrodynamic interactions among the particles and are neutrally stable at the linear level. Here, we analyze the effect of particle activity - self-propulsion - on the emergence and stability of these phonons. We show that the direction of wave propagation in active crystals is sensitive to the intensity of the background flow. We also show that activity couples, at the linear level, transverse waves to the particles' rotational motion, inducing a new mode of instability that persists in the limit of large background flow, or, equivalently, vanishingly small activity. We then report a new phenomenon of phonons switching back and forth between two adjacent crystals in both passively-driven and active systems, similar in nature to the wave switching observed in quantum mechanics, optical communication, and density stratified fluids. These findings could have implications for the design of commercial microfluidic systems and the self-assembly of passive and active micro-particles into one-dimensional structures.

Phonon-interference resonance effects by nanoparticles embedded in a matrix

We report an unambiguous phonon resonance effect originating from germanium nanoparticles embedded in silicon matrix. Our approach features the combination of phonon wave-packet method with atomistic dynamics and finite element method rooted in continuum theory. We find that multimodal phonon resonance, caused by destructive interference of coherent lattice waves propagating through and around the nanoparticle, gives rise to sharp and significant transmittance dips, blocking the lower-end frequency range of phonon transport that is hardly diminished by other nanostructures. The resonance is sensitive to the phonon coherent length, where the finiteness of the wave packet width weakens the transmittance dip even when coherent length is longer than the particle diameter. Further strengthening of transmittance dips are possible by arraying multiple nanoparticles that gives rise to the collective vibrational mode. Finally, it is demonstrated that these resonance effects can significantly reduce thermal conductance in the lower-end frequency range.

Davydov Collective Vibrational Modes and Infrared Spectrum Features in Aniline Crystal: Influence of Geometry Change Induced by van der Waals Interactions

Intermolecular interactions have significant influences on molecular crystals, oligomers, and various van der Waals clusters. They are essential in determining the quantum optics, quantum transport, and chemical properties of complex molecular systems. In this work, we investigate the infrared spectra of aniline crystal via the density functional theory. Then we identify four intriguing collective modes that are featured by NHHN wagging vibrations and four other collective modes that are featured by NH2 wagging vibrations. All these eight collective modes are due to Davydov splitting. To clarify the origin of such vibrational pattern, we further simulate aniline molecule and oligomers, and thoroughly analyze the spectra differences on some key vibrational modes, such as N–H wagging and torsional vibrations. Our results reveal that the chain structure of aniline crystal significantly enhances the van der Waals forces among adjacent molecules, and the intermolecular interactions are responsible for those NH...

Terahertz spectra of DNA nucleobase crystals: A joint experimental and computational study

Terahertz (THz) spectra of DNA nucleobase crystals were experimentally studied by terahertz time domain spectroscopy (THz-TDS), Fourier transform infrared spectroscopy (FTIR), and computationally studied by the generalized energy-based fragmentation approach under periodic boundary conditions (denoted as PBC-GEBF). We analyzed the vibrational spectra of solid-state DNA nucleobases and assigned the corresponding vibrational modes to the main peaks in the experimental spectra with the PBC-GEBF results. The computational results were verified to be in good accordance with the experimental data. Harmonic vibrational frequency results revealed that all the vibrational modes belong to collective vibrational modes, which involve complicated mixtures of inter- and intramolecular displacements, somewhere in the vicinity of 0.5–9THz.

Function of terahertz spectra in monitoring the decomposing process of biological macromolecules and in investigating the causes of photoinhibition

Chlorophyll a and β-carotene play an important role in harvesting light energy, which is used to drive photosynthesis in plants. In this study, terahertz (THz) and visible range spectra of chlorophyll a and β-carotene and their changes under light treatment were investigated. The results show that the all THz transmission and absorption spectra of chlorophyll a and β-carotene changed upon light treatment, with the maximum changes at 15 min of illumination indicating the greatest changes of the collective vibrational mode of chlorophyll a and β-carotene. The absorption spectra of chlorophyll a in the visible light region decreased upon light treatment, signifying the degradation of chlorophyll a molecules. It can be inferred from these results that the THz spectra are very sensitive in monitoring the changes of the collective vibrational mode, despite the absence of changes in molecular configuration. The THz spectra can therefore be used to monitor the decomposing process of biological macromolecules; however, visible absorption spectra can only be used to monitor the breakdown extent of biological macromolecules.

Insight into the collective vibrational modes driving ultralow thermal conductivity of perovskite solar cells

The past few years have witnessed a rapid evolution of hybrid organic-inorganic perovskite solar cells as an unprecedented photovoltaic technology with both relatively low cost and high-power conversion. The fascinating physical and chemical properties of perovskites are benefited from their unique crystal structures represented by the general chemical formula $AM{X}_{3}$, where the $A$ cations occupy the hollows formed by the $M{X}_{3}$ octahedra and thus balance the charge of the entire network. Despite a vast amount of theoretical and experimental investigations have been dedicated to the structural stability, electrical, and optical properties of hybrid halide perovskite materials in relation to their applications in solar cells, the thermal transport property, another critical parameter to the design and optimization of relevant solar cell modules, receives less attention. In this paper, we evaluate the lattice thermal conductivity of a representative methylammonium lead triiodide perovskite $({\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3})$ with direct nonequilibrium ab initio molecular dynamics simulation. Resorting to full first-principles calculations, we illustrate the details of the mysterious vibration of the methylammonium cluster $({\mathrm{CH}}_{3}{\mathrm{NH}}_{3}^{+})$ and present an unambiguous picture of how the organic cluster interacting with the inorganic cage and how the collective motions of the organic cluster drags the thermal transport, which provide fundamental understanding of the ultralow thermal conductivity of ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$. We also reveal the strongly localized phonons associated with the internal motions of the ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}^{+}$ cluster, which contribute little to the total thermal conductivity. The importance of the ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}^{+}$ cluster to the structural instability is also discussed in terms of the unconventional dispersion curves by freezing the partial freedoms of the organic cluster. These results provide more quantitative description of organic-inorganic interaction and coupling dynamics from accurate first-principles calculations, which are expected to underpin the development of emerging photovoltaic devices.

Sr2SmNbO6 perovskite: Synthesis, characterization and density functional theory calculations

The density functional theory (DFT) under the generalized gradient approximation (GGA) has been used to investigate the electronic structure of double perovskite oxide Sr2SmNbO6 synthesized by the solid-state reaction technique. The Rietveld refinement of the X-ray diffraction pattern of the sample shows the monoclinic P21/n phase at room temperature. The X-ray photoemission spectrum (XPS) of the material is collected in the energy window of 0–1200 eV. The chemical shift of the constituent elements calculated from the core level XPS spectra is used to analyze the covalency between the O anion and Sm/Nb cations. The valence band (VB) XPS spectrum is compared with the calculated VB spectrum using partial density of states in a standard way. The Raman spectrum is employed to investigate the phonon modes of the material in the monoclinic phase. Lorentzian lines are used to fit the experimental Raman spectrum, which present 24 phonon modes corresponding to the stretching and banding of NbO6/SmO6 octahedra and translational motion of Sr along the SrO bond. The discrepancy between the measured and calculated band gap values has been removed by applying modified Becke-Johnson (mBJ) potential in the DFT calculations. The experimental optical band gap obtained from the UV–visible reflectance spectrum is found to be 3.42 eV, which is well matched with the DFT calculated value of 3.2 eV, and suggests the semiconducting nature of the material. The real (ε′) and imaginary (ε″) parts of the optical dielectric constant as a function of energy along the x-, y- and z-polarization directions using mBJ potential are calculated. The collective vibrational modes of the atoms, the Born effective charge of the ions and their effect on the static dielectric constant of the material are studied using DFT. The calculated value of static dielectric constant for SSN is found to be 41.3.

Complex Contact-Based Dynamics of Microsphere Monolayers Revealed by Resonant Attenuation of Surface Acoustic Waves.

Contact-based vibrations play an essential role in the dynamics of granular materials. Significant insights into vibrational granular dynamics have previously been obtained with reduced-dimensional systems containing macroscale particles. We study contact-based vibrations of a two-dimensional monolayer of micron-sized spheres on a solid substrate that forms a microscale granular crystal. Measurements of the resonant attenuation of laser-generated surface acoustic waves reveal three collective vibrational modes that involve displacements and rotations of the microspheres, as well as interparticle and particle-substrate interactions. To identify the modes, we tune the interparticle stiffness, which shifts the frequency of the horizontal-rotational resonances while leaving the vertical resonance unaffected. From the measured contact resonance frequencies we determine both particle-substrate and interparticle contact stiffnesses and find that the former is an order of magnitude larger than the latter. This study paves the way for investigating complex contact-based dynamics of microscale granular crystals and yields a new approach to studying micro- to nanoscale contact mechanics in multiparticle networks.