Vibrational Excitation Spectra Research Articles

Role of the third dimension in searching for Majorana fermions in α−RuCl3 via phonons

Understanding phonons in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ is critical to analyze the controversy around the observation of the half-integer thermal quantum Hall effect. While many studies have focused on the magnetic excitations in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$, its vibrational excitation spectrum has remained relatively unexplored. We investigate the phonon structure of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ via inelastic neutron-scattering experiments and density-functional-theory calculations. Our results show excellent agreement between experiment and first-principles calculations. After validating our theoretical model, we extrapolate the low-energy phonon properties. We find that the phonons in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ that either propagate or vibrate in the out-of-plane direction have significantly reduced velocities and therefore have the potential to dominate the observability of the elusive half-integer plateaus in the thermal Hall conductance. In addition, we use low-energy interlayer phonons to resolve the low-temperature stacking structure of our large crystal of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$, which we find to be consistent with that of the $R\overline{3}$ space group, in agreement with neutron diffraction.

Synthesis, characterization, X-ray structural analysis, DFT and BSA binding study of a Zn(II) complex, [Zn(II)Cl2(nia)2].2nia

A Zn(II) complex with two nicotinamide (nia) ligands, two chloride ions, and two co-crystallized nicotinamide molecules, [ZnCl2(nia)2].2nia, was synthesized and characterized on the basis of elemental analysis, spectroscopic techniques (UV-vis, FT-IR, 1H and 13C NMR), conductivity measurements, DFT and single-crystal X-ray crystallography. The complex crystallized in orthorhombic crystal system with pccn space group and cell dimensions: a = 27.1852(2), b = 7.6705(4), c = 12.7502(8) Å and α = β = γ = 90°. Single-crystal structural analysis revealed the presence of a neutral complex having tetrahedral geometry. In this complex, Zn(II) is coordinated to two chloride and two nicotinamide ligands in the coordination sphere and two nicotinamide molecules are co-crystallized. Hydrogen bonding interactions (N-H…O, C-H…O, N-H…N, C-H…Cl) along with π…π bonding interactions (between the coordinated nicotinamide and free nicotinamide moiety) stabilize the crystal lattice. The DFT studies consolidated the experimental findings and provided deeper insight into vibrational spectra, excitation spectra and hydrogen bonding interactions between nicotinamide molecules. The interactions and binding parameters of this complex with bovine serum albumin (BSA) have been probed through absorption and fluorescence spectroscopy which revealed the presence of static quenching mechanism and good binding propensity of the Zn complex.

In Silico Studies on Sennidines-Natural Dianthrones from Senna.

Simple SummaryThe study determines the spatial structure and intramolecular interactions of sennidines—natural pharmaceutical substances present in Senna species. The calculations predict many sennidin conformers with similar energy but the gauche conformation will be present in the plant material. The lowest energy structure that is most likely to be found in plant material is characterized by the presence of OHO hydrogen bonds formed by hydroxyl groups and carbonyl oxygen. The sanidin molecule can be easily breakdown into monoanthrones because of elongation of the single C-C bond linking the anthrone moieties and reduced bond dissociation energy. The work contains data on theoretical, vibrational and electron excitation spectra, which can be used in the analysis of experimental samples.The rapid development of technology allows for more accurate research of biological systems with the use of in silico methods. One of the tools is the quantum-chemical method used for determining the structure, properties and interactions of molecules of great pharmacological importance. The accuracy of theoretical models is increasing and can be a real help in biology, chemistry and pharmacy. The aim of the study is to determine the spatial structure and intramolecular interactions of sennidines—natural pharmaceutical substances present in Senna species. Calculations carried out in the gas-phase and in the solvent model, compared with the available experimental data indicate the possibility of sennidines to form conformers. QTAIM and NCI analysis suggests the presence of many intramolecular interactions in the sennidin structure. Taking into account the lowest energy optimized structure, it can be predicted that the sennidin in the gauche conformation will be present in the plant material. The single C-C bond connecting the anthrone moieties is elongated and its reduced Bond Dissociation Energy (BDE) could be the cause of an easy breakdown of the sennidin molecule into monoanthrones. This work contains data on theoretical, vibrational and electron excitation spectra, which can be used in the analysis of experimental samples.

Energy Spectra Correlation of Vibrational and Electronic Excitations and Their Dispersion in Graphite and Graphene

The correlation between the vibrational and electron excitation modes in the energy spectra of single-layer graphene and crystalline graphite, as well as the dispersion dependences of those modes, has been studied. The methods of the theory of projective representations of the point and spatial symmetry groups are used for the first time in order to interpret those correlations. The correlations of vibrational and electron excitation spectra and the compatibility conditions for irreducible projective representations in the descriptions of quantum states of graphene and crystalline graphite at various points of their Brillouin zones are determined. For the projective representations of all projective classes belonging to the hexagonal system, standard factor-systems are constructed for the first time. In particular, the factor-systems for electron states are first determined. The results obtained are used to calculate, also for the first time, the correct spinor multiplication tables, i.e. the multiplication tables for elements in double symmetry groups. The developed method is applied to classify all high-symmetry points in the Brillouin zones of single-layer graphene and crystalline graphite with respect to the symmetry type of vibrational excitations.

Site selective Cu2+ substitution in single crystal Fe3O4 biocompatible nanospheres by solvothermal reflux method

Transition metal cations distribution among tetrahedral and octahedral sites in Fe3O4 has profound influence on it properties. The cations distribution among these sites were influenced by crystal field stabilization energy (CFSE) of a substituent ion. It was reported that Cu2+ substitutes both Fe3+ in tetrahedral and Fe2+ in octahedral sites of magnetite though it has high CFSE for octahedral sites. Thus it promotes mixed (normal & inverse) spinel structure and hence the properties deviate from the prediction. Here we show that selective octahedral site substitution of Fe2+ by Cu2+ in single crystal Fe3O4 nanoparticles was possible by solvothermal reflux method at moderate temperatures (≈300°C). It was attributed to reduced energy barrier for crystallization and high diffusion coefficient of the metal cations at moderate temperature in low viscous organic solvents mixture, at ambient pressure, compared to coprecipitation and solid state reaction protocols. In addition, effective removal of exothermic crystal lattice energy, released during nucleation and growth process, by natural gas bubbles formed in the reflux organic solvent mixture. This promotes effective diffusion of cations during the growth process and easy octahedral occupation by Cu2+ in Fe1−xCuxFe2O4 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5 & 0.6) single crystal nanospheres. The octahedral substitution was confirmed by reduction of crystal lattice parameter as well as ferromagnetic strength of octahedral sublattice with enhanced Cu2+ concentration. In addition, morphology, lattice vibrational frequency and electron excitation spectra of the nanospheres were studied. The Langevin function fit analysis reveals that superparamagnetic domain diameter were slightly less than the particle diameter obtained from TEM micrograph.

Effects of magnetic field and twin domains on magnetostructural phase mixture inMn3O4: Raman scattering studies of untwinned crystals

The ferrimagnetic spinel Mn3O4 exhibits large and anisotropic changes in electronic and structural properties in response to an applied magnetic field. These changes are thought to result from the field-dependent tuning---via strong spin-lattice coupling---between two nearly degenerate magnetostructural phases. Recent variable-magnetic-field studies of Mn3O4 have been performed on melt-grown crystals, which can exhibit twin domains due to a Jahn-Teller structural transition below the melting temperature. Because of the near degeneracy of the magnetostructural phases, however, strain associated with the twin domains likely affects the magnetic responses of Mn3O4. In this report, we present a variable-magnetic-field Raman scattering study of untwinned Mn3O4 crystals grown out of a flux below the Jahn-Teller structural transition. We measure distinct q = 0 magnetic and vibrational excitation spectra for each isolated magnetostructural phase of untwinned Mn3O4 crystals and determine the symmetries of the observed excitations. We determine how the magnetostructural phase mixture changes in response to magnetic fields applied in the magnetic easy plane. Lastly, by comparing results on flux- and melt-grown Mn3O4 crystals, we show that the intrinsic mixture of the two magnetostructural phases is indeed strongly influenced by the presence of twin domains.

Substituent effects on vibrational and electronic excitation spectra of pyridone tautomers and ions: The case of the cyano group

In this theoretical work, we computed the equilibrium geometries and a set of rotational and vibrational spectroscopic parameters for cyano substituted 2-pyridones neutral or cationic and their tautomers (cyano 2-hydroxypyridines). We examined also the effect of tautomerism equilibrium on those systems. In our analysis, we mostly focused on the perturbations induced by the CN group on the electronic structure and on the spectroscopy of 2-pyridone/2-hydroxypyridine block. Moreover, we investigated the pattern of their low lying electronic states at both the PBE0/aug-cc-pVDZ Density Functional Theory (DFT) and the CASSCF/aug-cc-pVTZ levels of theory. Vertical excitation spectra and both adiabatic and vertical ionization energies were performed.

Density‐functional‐based prediction of a spin‐ordered open‐shell singlet in an unpassivated graphene nanofilm

AbstractI briefly review some of the methods, developed at the Naval Research Laboratory, that were used in a variety of collaborations with the Frauenheim group. To aid experimental understanding at that time, we attempted to predict equilibrium structures and also provide calculated vibrational spectra for direct comparison to experimental measurements. Most of our work was on carbon‐based systems with a heavy focus on fullerenes and diamond. More recently another pure carbon compound, graphene, has received much attention. In this paper, I present applications of density‐functional‐based spectroscopic techniques to a single graphene nanofilm and confirm that spin polarization can occur at the edge states. However, antiferromagnetic ordering is preferred over ferromagnetic ordering. By extracting a Heisenberg Hamiltonian from the calculations the energy of the various spin multiplets can be estimated through diagonalization of this Hamiltonian. The Perdew–Burke–Ernzerhof–density‐functional theory (PBE‐DFT) results indicate that both antiferromagnetic and ferromagnetic graphene structures are vibrationally stable but that the lowest‐energy eigenstate of the system is an open‐shell singlet with a gap of approximately 14 cm−1 to the open‐shell triplet state. Consistent with recent work that stabilizes graphene nanofilms via adsorption onto metal surfaces the density functional results presented here show that small graphene nanofilms, while vibrationally stable, will exothermically form previously identified fullerene structures. Vibrational excitation spectra for the 54‐atom graphene and fullerene structures are calculated to provide spectroscopic means for distinguishing between the two structures.

TS-1 from First Principles

First principles studies on periodic TS-1 models at Ti content corresponding to 1.35% and 2.7% in weight of TiO(2) are presented. The problem of Ti preferential siting is addressed by using realistic models corresponding to the TS-1 unit cell [TiSi(95)O(192)] and adopting for the first time a periodic DFT approach, thus providing an energy scale for Ti in the different crystallographic sites in nondefective TS-1. The structure with Ti in site T3 is the most stable, followed by T4 (+0.3 kcal/mol); the less stable structure, corresponding to Ti in T1, is 5.6 kcal/mol higher in energy. The work has been extended to investigate models with two Ti's per unit cell [Ti(2)Si(94)O(192)] (2.7%). The possible existence of Ti-O-Ti bridges, formed by two corner-sharing TiO(4) tetrahedra, is discussed. By using cluster models cut from the optimized periodic DFT structures, both vibrational (DFT) and electronic excitation spectra (TDDFT) have been calculated and favorably compared with the experimental data available on TS-1. Interesting features emerged from excitation spectra: (i) Isolated tetrahedral Ti sites show a Beer-Lambert behavior, with absorption intensity proportional to concentration. Such a behavior is gradually lost when two Ti's occupy sites close to each other. (ii) The UV-vis absorption in the 200-250 nm region can be associated with transitions from occupied states delocalized on the framework oxygens to empty d states localized on Ti. Such extended-states-to-local-states transitions may help the interpretation of the photovoltaic activity recently detected in Ti zeolites.

Inelastic light scattering studies of overdoped Y1–xCax Ba2Cu3O7–δ thin film

AbstractThe vibrational and magnetic Raman excitation spectra were examined in overdoped Y1–xCaxBa2Cu3O7–δ thin films. Below Tc, certain phonons in undoped YBa2Cu3O7–δ (YBCO) show strong self‐energy effects, which gradually vanish with increasing Ca concentration into the overdoped regime. The observed B1g symmetry two‐magnon excitation peak near 2900 cm–1 in YBCO is significantly broadened, weakened, and shifts to the lower frequency with increasing Ca content, indicating the effective value of magnetic superexchange energy decreases and that the life time of the magnons becomes shorter with increasing hole concentrations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Determination of elastic properties and interatomic force constants using inelastic neutron scattering

Neutron scattering is the most powerful technique for measuring vibrational excitation spectra. With single‐crystal samples, the complete phonon dispersion relation can be measured. At small phonon wavelengths, the slope of the dispersion curve is related directly to the elastic constants of the material. Information from shorter wavelength phonons can be used to determine parameters for interatomic force models or compared to first‐principles calculations. Model parameters can then be used to calculate phonon densities‐of‐states and consequently connect to the thermodynamics. Polycrystalline samples yield less information, but the data is directly related to the phonon density‐of‐states for Bravais lattices. Measurements on polycrystalline samples have the advantage of being fast and are more useful for parametric studies of the lattice dynamics (with temperature, for example). These ideas will be used to discuss several interesting systems, most notably Pu metal and Fe‐Ga magnetostrictive alloys. [This work is supported by the U.S. Department of Energy.]

The role of vibrational doorway states in positron annihilation with large molecules

Positron annihilation rates in many polyatomic molecular gases are anomalously high. Qualitatively, this can be explained by positron capture in vibrational Feshbach resonances, which can occur for molecules with positive positron affinities [Gribakin, Phys. Rev. A 61 (2000) 022720]. To verify this idea quantitatively, we examine the densities of vibrational excitation spectra of alkanes. To understand the energy dependence of the annihilation rates for alkanes, we propose that positron capture is mediated by vibrational doorway states, in which positron binding is accompanied by the excitation of fundamentals.

The role of vibrational doorway states in positron annihilation with large molecules

Positron annihilation rates in many polyatomic molecular gases are anomalously high. Qualitatively, this can be explained by positron capture in vibrational Feshbach resonances, which can occur for molecules with positive positron affinities [Gribakin, Phys. Rev. A 61 (2000) 022720]. To verify this idea quantitatively, we examine the densities of vibrational excitation spectra of alkanes. To understand the energy dependence of the annihilation rates for alkanes, we propose that positron capture is mediated by vibrational doorway states, in which positron binding is accompanied by the excitation of fundamentals.

Potential energy surface of the CO2? anion

Electron attachment to CO2 is studied by exploring the parts of the potential energy surface where an electron can be bound in the fixed-nuclei picture. High level ab initio methods are used, and in particular the question of an adequate description of the threshold region, where the attachment energy goes to zero and the electronically bound anionic states become unstable, is discussed. Since the relevant coordinates are the symmetric stretching and the bending mode, we consider a two dimensional cut (C2v symmetry constraint) through the three dimensional nuclear coordinate space, and specifically C–O bond lengths between 1.1 and 1.6 Å and bending angles between 180 and 125° are examined. Three bound CO2− states are identified in this region. The minimum associated with the long-lived CO2− state is localised on the lowest surface of the anion, and only a small barrier separates it from the region where the anion becomes unstable. Using earlier results from R-matrix calculations and our findings we can then answer the question which of the short-lived scattering states connects to the long-lived CO2− species. All three states of the CO2− anion are vibronically coupled, and the implications for the nuclear dynamics of these states are discussed, in particular in view of the recently observed vibrational excitation spectra.

Reactive vibrational excitation spectroscopy of formic acid in solid argon: Quantum yield for infrared induced trans→cis isomerization and solid state effects on the vibrational spectrum

Formic acid molecules are trapped in two predominant local environments (sites) when isolated in an argon matrix at 8 K. Using narrowband tunable infrared (IR) radiation, we performed site-selective excitation of various vibrational modes of the lower-energy trans conformer. For all excited modes, ranging from 7000 to 2950 cm−1, we detected site-selective isomerization to the higher-energy cis form. By measuring the IR absorption of a selected band of the cis conformer as a function of the excitation frequency, the reactive vibrational excitation (RVE) spectra were obtained. The trans→cis isomerization quantum yields for the excited modes were determined. Remarkably, very high absolute values were obtained for the quantum yield (up to 40%) at excitation energies above the reaction barrier. The efficiency of the photoinduced isomerization is essentially independent of the excited vibrational mode in a broad energy interval. Even when the excitation energy was below the reaction barrier, IR-induced rotational isomerization was observed, which indicates tunneling from the vibrationally excited trans conformer to the cis form. Using the RVE spectra, phonon sidebands were detected on the high-frequency side of the zero-phonon-line of the OH stretching mode of trans-formic acid. These weak and broad bands were not observed in the absorption spectra. Additionally, a relatively narrow band blueshifted by 6 cm−1 from the OH stretching fundamental mode was assigned to a librational satellite based on simulations using the hindered rotation model for an asymmetric top trapped in an octahedral crystal field.

Laser excitation spectrum and spectroscopic potential parameters of Cd 2 molecule in the 1 u(5 3P 2) energy state

Excitation spectrum of Cd 2 van der Waals complex was observed in a supersonic free-jet expansion beam. Cadmium dimers were seeded in argon environment (a carrier gas) while a pulsed dye laser served as an excitation light source. Well resolved structures of vibrational bands arising from transition between 1 u( 3Π u) and X0 g +( 1Σ g +) states were observed. Using a Birge–Sponer method for analyzing the vibrational excitation spectrum, the first-time determination of the spectroscopical potential parameters of the 1 u molecular state was performed. We obtained D′ e=723±10 cm −1, ω′ e=28.9±1.0 cm −1, ω′ e x′ e=0.260±0.002 cm −1 and R′ e=3.93±0.05 Å. The latter value was estimated with the help of the computer-simulation of the spectrum. The results are compared with recent results of ab initio calculation of Czuchaj et al. (Chem. Phys. Lett. 225 (1994) 233) and demonstrate a reasonable agreement.

HREELS study of C 70 molecules adsorbed on a Si(1 1 1)-(7×7) surface

We have measured the coverage dependence of vibrational excitation spectra of C 70 molecules adsorbed on a Si(1 1 1)-(7×7) surface using high-resolution electron-energy-loss spectroscopy. At the monolayer coverage, the intensity of the 57 meV peak increases, and those of the 83 and 178 meV peaks decrease. Taking into account the dipole selection rule, the change in intensity of the 57 meV peak indicates that the average angle between the long axes of C 70 molecules and surface normal is about 40°. The decreases in intensities of the 83 and 178 meV peaks suggest that the rotational motion of molecules is quenched upon adsorption. We will discuss the Coriolis interaction between the accidentally degenerate A 2″ and E 1′ modes.

Normal modes of vibration of the C60 isotopomers C5912C13 and C5812C213

The vibrational excitation spectra of the isotopomers C5912C13 and C5812C213 has been calculated within the framework of a first principle calculation. The force constants between the carbon atoms we obtained by performing the second derivatives of the energy within the Hartree–Fock approximation. A comparison of the frequency shifts in the vibrational spectra produced by the isotopes with Raman spectroscopy experiments is also presented.

Interaction of C 60 with Si(111)7×7 and Si(100)2×1 surfaces studied by STM, PES and HREELS: annealing effect

We report here measurements of the valence band spectra, the C 1s core level spectra and the vibrational excitation spectra of a C 60 monolayer (ML) film on Si(111)7×7 and Si(100)2×1 surfaces, using photoelectron spectroscopy and high-resolution electron energy loss spectroscopy (HREELS) in combination with scanning tunneling microscopy. 1 ML films are formed after annealing the 5 ML films at 670 K. The bonding states between C 60 molecules and Si substrates are clearly observed at 2.1 eV on both surfaces in the valence band spectra. The shift of binding energies of the molecular orbitals and the C 1s core level spectra indicate the charge transfer. The softening of several vibrational modes is observed on the Si(111)7×7 and Si(100)2×1 surfaces with HREELS. The charge transfer scheme explains the softening well. We discuss the bonding nature in terms of hybridization of C 60 molecular orbitals with the surface states.

Bonding nature of C 60 adsorbed on Si(111)7×7 and Si(100)2×1 surfaces studied by HREELS and PES

We report here the measurements of vibrational excitation spectra, valence band spectra and C 1s core level spectra of C 60 molecules adsorbed on Si(111)7×7 and Si(100)2×1 surfaces, using high-resolution electron energy loss spectroscopy (HREELS) and photoelectron spectroscopy (PES). At a coverage between 1 and 0.25 ML, HREELS and PES measurements show that the interaction is as weak as the van der Waals interaction on both surfaces. At a coverage lower than 0.25 ML, some contradictory results are obtained by HREELS and PES. With HREELS, the softening of several vibrational modes is observed on the Si(111)7×7 surface, but no indication of softening is observed on the Si(100)2×1 surface. The charge transfer scheme explains the softening well. These results by HREELS indicate that the nature of the bonding is ionic on the Si(111)7×7 surface and of the van der Waals type on the Si(100)2×1 surface. In contrast, the bonding states between C 60 molecules and Si substrates are clearly observed at 2.4 eV on both surfaces with PES. The PES measurements indicate that the interaction has a covalent character on the Si(111)7×7 surface, and that it is in between covalent and ionic, i.e. not purely ionic or at the covalent limit, on the Si(100)2×1 surface. We will discuss the nature of the interaction manifested by HREELS and PES measurements in comparison with the spectra of C 60 molecules adsorbed on metal surfaces.