On surface phonons and reconstruction: Comparison of the W and the Fe (001) surface

A phenomenological surface phonon model of the (001) surface of W and Fe is used to clarify some questions related to the problem of which high-symmetry surface phonon mode is the most perspective candidate for the tungsten (001) surface reconstruction observed experimentally. Conditions are derived for the appearance and softening of the relevant modes. The ¯M ₁,¯M ₅ and ¯X ₃ modes seem to be the most promising. A few points from the current literature on the reconstruction problem are discussed. The results seem to support the idea about the general instability of the tungsten (001) surface.     

Morphology of InN nanorods using spectroscopic Raman imaging

We report the vibrational properties of vertical and oblique InN nanorods (NRs) grown by molecular beam epitaxy (MBE). Surface optical (SO) Raman mode at 561 cm⁻¹, belonging to E₁ symmetry [SO(E₁)], is identified along with symmetry allowed Raman modes of E₂(low), E₂(high), and E₁(LO) at 87, 489, and 589 cm⁻¹, respectively, corresponding to wurtzite InN phase. Usually, SO phonon modes arise due to breakdown of translational symmetry of surface potential at surface defects, which are attributed by the surface roughness. Intensity distribution of E₁(LO) and SO(E₁) phonon modes over a specified area have been analysed using Raman area mapping with an optical resolution of 400 nm. Imaging with E₁(LO) phonon mode, originating from the bulk of the sample, distinguishes the vertical NRs alone. We are able to resolve NR morphologies in both vertical and oblique cases with additional Raman mapping analysis of SO(E₁) phonon mode, emerging from the surface irregularities, which are confined to the tip of MBE grown NRs. Copyright © 2013 John Wiley & Sons, Ltd.     

Disorder‐induced Raman scattering effects in one‐dimensional ZnO nanostructures by incorporation and anisotropic distribution of Dy and Li codopants

In Dy³⁺ and Li⁺ codoped ZnO nanowires, the additives accumulate preferentially in {0001} planes, resulting in serious breakdown of the translational symmetry in ab plane and modification of the phonon oscillation field. Not only acoustic overtones, silent optical modes, surface optical (SO) phonon modes, and multi‐phonon processes can be effectively observed in the nonresonant Raman scattering (RS) and the Fourier‐transform infrared (FTIR) spectra, but the quasi‐LO and TO modes of mixed A₁ and E₁ symmetry also show a noticeable red shift from E₁ symmetry (in ab plane) to A₁ symmetry (along c axis). The presence of dislocations and internal strain at the surface layer rich in additives, coming from the segregation of additives, forms a quasi‐bilayer system, resulting in the appearance and enhancement of SO phonon modes in RS and FTIR spectra. The Fano interference, originating from the interaction between the discrete scattering from phonons and the continuum scattering from laser‐induced electrons in the doped nanostructures, leads to typical asymmetric lineshapes on the lower wavenumber sides. Copyright © 2010 John Wiley & Sons, Ltd.     

The (2 × 2) reconstructions on the SrTiO3 (001) surface: A combined scanning tunneling microscopy and density functional theory study

Scanning tunneling microscopy study showed that the (2 × 2) reconstruction on the (001) surface of SrTiO₃ should have a surface structure with a 4-fold symmetry. The previously proposed solution for the (2 × 2) reconstruction with the p2gm symmetry only has a 2-fold symmetry. In this study density functional theory study was carried out to propose a possible surface structure with the p4mm surface symmetry which matches the scanning tunneling microscopy images and suggests that two different (2 × 2) surface structures exist. The formation of the (2 × 2) reconstruction with the p4mm symmetry may be due to the kinetics as it has slightly higher surface energy than the one with the p2gm symmetry.     

Surface‐enhanced Raman spectroscopy of indanthrone and flavanthrone

The normal Raman and surface‐enhanced Raman scattering (SERS) spectra of flavanthrone and indanthrone were obtained at several excitation wavelengths. The spectral assignments were aided by density functional calculations. Since both molecules have very high symmetry (C_2h) including a center of inversion, we expect that the modes of u symmetry will be forbidden in the normal Raman spectrum. However, proximity to the surface causes special SERS enhancement of several of the b_u modes, along with somewhat weaker enhancement of the a_u and b_g modes. Copyright © 2009 John Wiley & Sons, Ltd.     

The adsorption of ammonia on a Fe(110) single crystal surface studied by high resolution electron energy loss spectroscopy (EELS)

EELS spectra of ammonia adsorbed on a Fe(110) single crystal surface at 120 K reveal four different molecular adsorption states:1. At very low exposures (0.05 L) three vibrational losses at 345 cm^?1, 1170 and 3310 cm^?1 are observed which are attributed to the symmetric Fe-N stretching-, N-H₃ deformation and N-H₃ stretching modes of chemisorbed molecular ammonia, respectively. The observation of only three vibrational losses indicates an adsorption complex of high symmetry (C_3v).2. Further exposures up to 0.5 L cause the appearance of additional losses at 1450 cm^?1, 1640 cm^?1 and 3370 cm^?1. The latter two are interpreted as the degenerate NH₃ deformation and - stretching modes of molecularly adsorbed NH₃. The 1450 cm^?1 loss is a combination of the losses at 345 cm^?1 and 1105 cm^?1. The observation of 5 vibrational losses is consistent with an adsorption complex of C_s symmetry.3. In the exposure range from 0.5 to 2 L adsorption of molecular ammonia in a second layer is observed. This phase is characterized by a symmetric deformation mode at 1190 cm^?1 and by two additional very intense modes at 160 cm^?1 and 350 cm^?1 which are due to rotational and translational modes.4. Exposures above 2 L cause multilayer condensation of ammonia characterized by translational and rotational bands at 190 cm^?1, 415 cm^?1 and 520 cm^?1, and a symmetric deformation mode at 1090 cm^?1. A broad loss feature around 3300 cm^?1 is attributed to hydrogen bonding in the condensed layer.Thermal processing of a Fe(110) surface ammonia covered at 120 K leads to decomposition of the ammonia into hydrogen and nitrogen above 260 K. No vibrational modes due to adsorbed NH or HN₂ species were detected.     

Internal modes and the local symmetry of PO4 tetrahedra in K(H1−xDx)2PO4 by Raman scattering

The v₃ and v₄ internal modes of PO^3?₄ tetrahedra are observable in KH₂PO₄ even above the transition temperature in the scattering geometry x(zz)y contrary to the selection rule of the site symmetry S₄ in the D_2d space group. Tominaga et al. discussed that the origin of this broken selection rule is due to the momentary site symmetry C₂ of these tetrahedra, and that the mechanism of the phase transition is of the order-disorder type of these distorted tetrahedra. Parallel studies compatible with their results have been carried out on mixed crystals K(H_1?xD_x)₂PO₄ and the same conclusion has been obtained as to the momentary site symmetry. These internal modes increase in intensity with deuteration, and the origin of this increase is discussed.     

Vibrational spectroscopy of tolane; Coriolis coupling between Raman-active modes of g symmetry

ABSTRACT

Vibrational spectroscopy of tolane (diphenylacetylene), which has 66 normal modes, has been advanced. Anharmonic wavenumber predictions were made with the quartic potential energy surface obtained with B3LYP/cc-pVTZ model and the second-order perturbation theory (VPT2). Infrared (IR) intensity and Raman activities were computed at the harmonic level. The IR spectrum of the crystal and Raman spectra of the liquid and the crystal tolane were newly recorded. The lingering problem of an excess of polarised Raman bands at wavenumbers appropriate for fundamentals, other than a_g modes, has now been attributed to Coriolis coupling within modes of g symmetry species. Consequently, D_2h point symmetry group has been confirmed for a planar tolane molecule. Assignments for almost all fundamentals of tolane are now secure. The assignment for ν₃₂ remains questionable. Remaining unassigned fundamentals are: ν₃₄ and ν₃₅, which, as a_u symmetry species, are IR- and Raman-inactive transitions, and ν₅₉(b_2u), which is predicted to have a very low wavenumber.     

Relativistic Jahn–Teller Effect for Triplet States of Tetrahedral Molecular Complexes

A relativistic multimode Jahn?Teller effect for tetrahedral molecular complexes in a triplet electronic state is considered. The analysis is based on the symmetry properties of the electronic Hamiltonian and its generalized symmetry operators, acting on both the coordinates (spatial operations) and spins (matrix operations) of the electrons. As a result, a 9 × 9 vibronic matrix that includes the vibronic coupling constants of orbital and spin-orbital nature and depends on the five normal modes of t₂ and e symmetry has been obtained.     

Adsorption of CO on W(001) by high resolution electron spectroscopy

The vibrational modes induced by CO on W(001) at temperatures ? 350 K are detected by means of electron energy loss spectroscopy with resolution in the 6–7 meV range. Two β adsorption regimes are identified depending on coverage. Heating at various increasing temperatures reveals coverage dependant irreversible surface structure modifications. The β spectra after adsorption or desorption are discussed in terms of the usual questions of multiple β states, dissociation, and reconstruction. The α₁ and α₂ states are detected both by their WC and CO frequencies. A small signal is assigned to a new a-state, named α₃, which may explain some thermal desorption results.     

SrTiO3(001)-c(6 × 2): a long-range,atomically ordered surface stable in oxygen and ambient air

We report about a new reconstruction of the SrTiO₃(001) surface which exhibits a c(6 × 2) unit cell, shows remarkable long range order, covers the entire surface exclusively and is stable in O₂ and even in ambient air. It is generated by a combination of oxygen and UHV annealing and characterized by low energy electron diffraction and atomically resolved scanning tunneling microscopy. Two rotationally equivalent domains of this reconstruction exist on the (001) surface, which exhibits fourfold symmetry. However, the c(6 × 2) structure can be stabilized as single domain on vicinal (001) surfaces.     

On the study of the vibrational energy levels of Arsine molecule

We compare two formalisms applied to the vibrational modes of the molecule of AsH₃ of C_3v molecular symmetry group. Indeed, the close stretching modes of this molecule may be considered as those of a three-dimensional oscillator whereas the bending modes may be considered either as a one-dimensional oscillator of symmetry A₁ and a two-dimensional oscillator of symmetry E or as an approximate three-dimensional oscillator. So, we have applied the U(p + 1) formalism to the both stretching and bending modes and introduced coupling terms acting on an appropriate coupled vibrational basis through a local mode formalism. We have then compared the result of our fitting with those obtained with the coupling of a local mode formalism adapted to the stretching vibrations with a normal mode formalism for the bending ones. Finally we compare our results with other methods recently proposed in the literature.     

The interplay between the surface band structure and possible surface reconstructions of Mo(112)

The experimental band structure of Mo(112) and the effects by temperature and adsorbate are presented. A surface resonance, identified as crossing the Fermi level at about 1/3 from to of surface Brillouin zone, was observed to be very sensitive to both contamination and temperature. We find evidence of adsorbate and temperature induced reconstruction of the Mo(112) surface. Examination of low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) data provides evidence for an adsorbate induced reconstruction of the Mo(112) surface with periodicities consistent with the Fermi level crossing of the surface resonance. The reconstruction is found to occur at coverages as low as 0.03 Langmuirs of oxygen or carbon. The reconstruction and/or adsorbate affects the density of states and bands near the Fermi level of a ₁ symmetry. Received 3 March 1999 and Received in final form 1 October 1999     

Temperature and composition dependence of vibrational modes in Sm1−xLaxAlO3

At room temperature and for x₀ ～ 0.43, the system Sm_1?xLa_xAlO₃ undergoes a first order phase transition in which an orthorhombic structure transforms into a rhomboedral structure as x increases. This phase transition was investigated by Raman scattering experiments on polycrystalline compounds in the range 0 ? x ? 1. The dependence of low frequency modes on x is reported and compared with their well known temperature dependence. Soft modes have been observed in the orthorhombic (D_2h¹⁶) and rhomboedral (D_3d⁶) phases. It was found that the first order phase transition is probably driven by a double degenerate mode whose two components in the low symmetry phase display a linear composition dependence of their squared frequencies. The source of anomalous behaviour of Raman bands near the phase transition is discussed.     

Raman scattering from (110) Hg0.8Cd0.2Te.

We report the first polarization dependent Raman scattering from the (1$\text{1}$0) surface of Hg_0.8Cd_0.2Te. In addition to the “HgTe-like” and “CdTe-like” transverse and longitudinal $\text{(}\text{q}\text{=0)}$ optical modes, we have observed a “clustering mode” [Γ₁ symmetry], a feature due to a coupled LO phonon-intersubband excitation in the surface inversion layer and a symmetry forbidden TO feature which may be related to internal strain. The previously reported “defect mode” was not observed.     

Softening of charge density wave excitations at the superstructure transition in 2H-TaSe2

Raman scattering measurements performed between 5 K and 300 K on 2H-TaSe₂ reveal new modes which are assigned to the modes of the charge density wave, observed in light scattering due to the Fermi surface induced distortion. The mode at 49 cm^?1 of E_2g symmetry softens (with concurrent line-width broadening) towards 122 K, the transition temperature from the incommensurate distorted to the undistorted phase. The mode at 82 cm^?1 of A_1g symmetry appears to be connected with the transition at 90 K from the commensurate to the incommensurate superstructure. The mode at 24.5 cm^?1 of E_2g shows no temperature dependence and is clearly due to the rigid-layer vibration.     

I.R. absorption due to H− and D− Ions in BaClF and SrClF

The i.r. absorption spectra sssociated with the vibrations of H^? and D^? ions in SrClF and BaClF have been studied in the range 600–2000cm^?1.The U centers are in fluorine sites. Because of the D_2d symmetry of the fluorine site, the triple degeneracy of the U center local mode in crystals of cubic symmetry, is split in BaClF and SrClF. In these crystals (space group D⁷_4h) unlike what happens with the cubic crystals, the symmetry types of the lattice band modes which give rise to the local mode sidebands, can be known experimentally. These band modes are almost only of E symmetry type. The E symmetry modes involve a vibration of the U center.A calculation of the local mode frequencies in BaClF:H^? from the experimental one in BaF₂:H^?, has been performed. An H^? ion shell model is more adequate than a rigid ion model to account for the frequencies and the relative intensities of the localized vibrational lines. The best agreement is obtained when the shell charge and the crystal polarizability of the H^? ion in BaF₂ are respectively ?1.3 e and 2.6 ų.     

Modes of vibrations due to (U-center + additive cation impurities) in alkali halides

Local and resonant modes due to hydride ions in various alkali halides containing additive cation impurities have been computed by the Green function technique. Local vibrations due to U-centers in alkali halides have O_h symmetry. When one of the six nearest neighbour cations is replaced by an additive impurity, the site symmetry of the system is lowered from O_h to C_4v. The phonon Green functions matríx is analysed according to the irreducible representation of the point group symmetry pertaining to the substitutional impurity. We have considered the vibrations of the hydride ion and all its six nearest neighbours. Analytical expressions have been derived for various modes of vibrations. Using group theory the 21 × 21 matrix has been block diagonalized into various irreducible representations. The effect of mass changes and the changes in short-range force constants have been taken into account. The computed results of the localized modes have been compared with the available experimental results. Good agreement has been found. Theoretical results on resonant modes are also displayed, which will be of use in future experiments on these systems.     

The quasi-confined optical phonons in wurtzite GaN/AlN multiple quantum wells

Within the framework of the dielectric-continuum model and Loudon's uniaxial crystal model, the equation of motion for p-polarization field in arbitrary wurtzite multilayer heterostructures are solved for the quasi-confined phonon (QC) modes. The polarization eigenvector, the dispersion relation, and the electron-QC interaction Fröhlich-like Hamiltonian are derived by using the transfer-matrix method. The dispersion relations and the electron-QC coupling strength are investigated for a wurtzite GaN/AlN single QW. The results show that there are infinite branches of dispersion curve with definite symmetry with respect to the center of the QW structure. The confinement of the quasi-confined phonons in the QW leads to a quantization of q_z,j characterized by an integer m that defines the order of corresponding quasi-confined modes. The QC modes are more dispersive for decreasing m. The QC modes display an interface behavior in the barrier and a confined behavior in the well. The symmetric modes have more contribution to electron-QC interaction than the antisymmetric modes. The strains have more effect on symmetry modes, and can be ignored for symmetry modes.     

Coupling to circularly asymmetric modes via long-period gratings made in a standard straight fiber

We demonstrate long-period fiber gratings made in a straight standard optical fiber via a CO₂ laser side irradiation that couples light between modes of different azimuthal symmetry. We show that such coupling can be even stronger than that to the azimuthally symmetric modes, provided the photoinduced asymmetry in the refractive index change across the fiber cross-section is high enough. This suggests that some previously-observed phenomena with CO₂-written LPGs not observed with other types of LPGs may be attributed to LPG resonances that couple light into the cladding modes with higher azimuthal symmetry. We show it on an example of bend-induced LPG resonance position tuning with the resonance depth maintained constant.