Surface Vibrational Modes Research Articles

Vibrational properties of the reconstructed W(100) surface from total energy results

A force constant model for the dynamical matrix of the clean low temperature reconstructed W(100) surface has been developed based on recent total energy results. The surface has been found to exhibit a number of interesting surface vibrational modes and resonances including three modes which are folded back by the surface reconstruction. These results have interesting implications for EELS and He atom scattering data.

Surface phonons of Ag(110): The importance of odd-symmetry modes in seeking accurate interaction models

Inelastic electron scattering measurements and lattice dynamical analysis of the vibrational properties of Ag(110) surfaces are presented. The measurements include detection of odd-symmetry shear horizontal modes throughout the two-dimensional Brillouin zone, and the lattice dynamical analysis explores the effects on surface interaction models arising from the requirement that shear mode properties are correctly accounted for by the model. This new constraint leads to a lattice dynamical model which is significantly different from models which are deduced on the basis of only evensymmetry surface vibrational modes.

Bulk and surface vibrational modes in NiAl.

We have measured the phonon dispersion curves of bulk NiAl along the three major symmetry directions by neutron scattering, and fitted the results with force models extending to third- and fourth-nearest neighbors. Our electron-energy-loss experiments show that dipole-active vibrational modes occur in the gap between the bulk acoustical and optical modes for the (110) and (111) surfaces. We have done extensive slab calculations of the vibrational modes at the surface-Brillouin-zone center for these two faces and for NiAl(001). The calculations indicate that the atomic motion for the gap modes is correlated with the bulk properties. Adjustment of the interplanar force constants in directions intuitively consistent with experimental results for the surface relaxations can shift the calculated frequencies into agreement with experiment.

Localized vibration modes of H adsorbed Si(111) surface

The surface-phonon dispersion curves of H adsorbed Si(111) surface have been calculated with force constants determined from a semiempirical force constant model. A variety of interesting surface vibrational modes are found and discussed for this surface. The relation between surface structure and surface vibrational modes is further elucidated by comparison with similar modes on D- and T- adsorbed surfaces as well as vibrational modes found on ideal Si(111) surface.

Folded, confined, interface, surface, and slab vibrational modes in semiconductor superlattices

Vibrational modes of semiconductor superlattices are related to those of their bulk constituents. Reduction in the translational symmetry enables observation by optical spectroscopy of symmetry forbidden modes of the bulk materials. Interesting but subtle concepts, such as Brillouin zone folding and mode confinement, arise. Moreover, electrostatic interface phonons exist in ideal infinite superlattices. In practical superlattices with a finite number of periods, slab and surface vibrational modes also occur. Most of the past experiments have been performed by means of Raman spectroscopy on GaAs Al xGa 1−xAs superlattices; recent work on Ge Si systems reveals new features. The properties of these modes, their symmetries and relationship to the modes of the bulk components are discussed.

Low-frequency raman scattering on surface vibrational modes of microcrystals

Low-frequency peaks have been observed in Raman spectra of nucleated glasses. A phenomenological model is proposed for their explanation. The data obtained from the analysis of Raman spectra are compared with the results of small-angle X-ray scattering. It is shown that the low-frequency Raman spectra permit to determine conditions at the boundaries, sizes and concentrations of structure microinhomogeneities in disordered materials.

Inelastic electron-tunneling study of MgO barriers.

A detailed study of MgO under a variety of preparation conditions has been carried out by inelastic electron-tunneling spectroscopy on Mg-MgO-Pb junctions. Surface vibrational modes play an important part in the observed spectra. The strong peak at 83 meV can be associated with the excitation of longitudinal-optical surface modes while two smaller maxima at 54 and 69 meV are attributable to a mixture of longitudinal- and transverse-optical surface modes. Some barriers were implanted with low-energy hydrogen ions creating magnesium hydride at the Mg-MgO interface. Hydrogen, either as magnesium hydride or substitutional ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ in the MgO lattice, was observed as an intense broad band near 124 meV in the inelastic tunneling spectrum. The catalytic action of water and hydrogen in the oxide growth process and their effect on the barrier height and roughness are also discussed.

New type of surface atomic vibrational modes of the (100) surface of the NaCl-type lattice.

On the basis of a lattice-dynamical calculation using the nearest-neighbor force-constant model of a 15-layer slab, we have discussed the optical and acoustical surface phonons of the (100) surface of the NaCl-type lattice. The stiffening of the in-plane force constant of the outermost layer has caused drastic changes of the surface phonon modes associated with the parallel vibrations to the surface. Instead of Lucas modes, the stiffening of 20% results in a new type of surface vibrational modes, which explains qualitatively the recent experimental dispersion curves of the longitudinal surface phonons of the TaC(100) and NbC(100) surfaces.

Assignment of a surface vibrational mode by chemical means: Modification of the lattice modes of Al2O3 by a surface reaction with H2O

The adsorption of water on a well-characterized Al2O3 film produced by oxidizing an Al(111) surface has been studied using electron energy loss spectroscopy (EELS). Water reacts with this oxide layer to form surface hydroxyl species characterized by a sharp O–H (O–D) stretching vibration at 3720 (2740) cm−1. The assignment of the three-peak vibrational spectrum of Al2O3 to surface and bulk Al–O modes is confirmed by chemical means based on the observation that the 625 cm−1 loss feature assigned to a surface Al–O stretching vibration is preferentially affected upon formation of surface hydroxyl groups. The remaining two bulk modes are uninfluenced by surface hydroxylation. The surface hydroxyl species can be quantitatively removed by electron stimulated desorption (ESD), reversing the spectroscopic changes observed upon exposure of Al2O3 to water, further substantiating the vibrational assignments.

Vibration eigenmodes and size of microcrystallites in glass: Observation by very-low-frequency Raman scattering.

The observation of very-low-frequency bands by Raman scattering in a nucleated cordierite glass is described. The frequency of the maximum of scattering is proportional to the inverse diameter of the particles, which are spherical spinel microcrystallites. It is shown that vibrational surface modes of particles are responsible for this Raman scattering.

Resonant and kinematical enhancement of He scattering from LiF(001) surface and pseudosurface vibrational normal modes.

One-phonon cross sections calculated from sagittally polarized vibrational normal modes account for most salient inelastic-scattering intensities seen in He-LiF(001) 〈100〉 and 〈110〉 measurements published by Brusdeylins, Doak, and Toennies. We have found that most inelastic intensities which cannot be attributed to potential resonances can be explained as kinematically enhanced scattering from both surface and pseudosurface bulk modes.

Surface green function matching for a three-dimensional nonlocal continuum

With a view toward helping to bridge the gap, from the continuum side, between discrete and continuum models of crystalline, elastic solids, explicit results are presented for nonlocal stress tensors that describe exactly some lattice dynamical models that have been widely used in the literature for cubic lattices. The surface Green function matching (SGFM) method, which has been used successfully for a variety of surface problems, is then extended, within a continuum approach, to a nonlocal continuum that models a three-dimensional discrete lattice. The practical use of the method is demonstrated by performing a fairly complete analytical study of the vibrational surface modes of the SCC semi-infinite medium. Some results are presented for the [100] direction of the (001) surface of the SCC lattice.

Surface vibrational properties

An overview is given of the principal types of surface vibrational modes that occur at a clean surface of a solid and the means whereby they can be studied experimentally. The modes that are discussed include long-wavelength surface acoustic modes (Rayleigh waves), short-wavelength surface acoustic modes, long-wavelength surface optical modes (surface polaritons), and short-wavelength surface optical modes. Typical dispersion curves illustrating the various types of modes are presented. Mention is also made of surface vibrational modes associated with adsorbed species on solid surfaces. Experimental techniques that are discussed include inelastic electron and helium atom scattering, Raman and Brillouin scattering, acoustic wave reflection, and attenuated total reflection. Other vibrational properties considered are surface specific heat and surface mean square displacements (Debye–Waller factor).

Surface reconstruction and lattice dynamics of hydrogenated Si(001):2 x 1.

We present a realistic calculation of the equilibrium surface structure and vibrational excitations of the hydrogenated Si(001):2\ifmmode\times\else\texttimes\fi{}1 surface. We study a saturation monohydride coverage which is found to produce a (2\ifmmode\times\else\texttimes\fi{}1) surface of symmetric dimers. A variety of interesting surface vibrational modes is found and discussed for this surface including a band of high-frequency vibrations associated with fivefold rings formed by the surface reconstruction. The relation between surface structure and surface vibrational modes is further elucidated by comparison with similar vibrational modes found on clean reconstructed Si(001):2\ifmmode\times\else\texttimes\fi{}1. The results for Si-H bond bending and stretching frequencies agree with experimentally measured values for this surface.

Theory of electron energy loss spectroscopy at the surface of a planar stratified medium, application to semiconductor superlattices

The nonretarded, macroscopic surface and interface optical vibrational modes of a layered material are theoretically investigated via their electron energy loss spectrum (EELS) in the reflection geometry. For this purpose, a continued-fraction representation of the relevant effective dielectric function of an arbitrarily stratified medium is obtained. For a semi-infinite superlattice two distinct types of vibrational excitations are predicted. The first gives rise to weak EELS continua and consists in Bloch-like interface modes propagating throughout the layered medium. The second type is responsible for strong EELS lines outside the above continua and is associated with evanescent, Fuchs-Kliewer-like surface modes of the superlattice. Application to a model GaAs/InAs superlattice is given for illustration.

Effects of Adsorption Site Symmetry on Vibrations at Metal Surfaces

It is shown how results from an efficient calculational scheme for lattice dynamics of dipole active surface vibrational modes, in the energy range of the bulk phonon band, can be used to disciminate between different adsorption sites on a metal surface. In particular, the comparison of results for the ordered structures c(2 × 2)CO- and p(2 × 2)S-Cu(100) with measured Electron Energy Loss (EEL) spectra yields unique determination of the adsorption sites in the two cases. These results are readily interpreted in terms of symmetry restrictions for the dipole activity of surface phonons. In addition, sum rules for EEL spectra are derived and discussed.

I.r. spectroscopic evidence for surface vibrational modes formed upon dehydroxylation of alumina

Dehydroxylation of alumina at an activation temperature (Tac) >673 K gives rise to a band in the 1020–1050 cm–1 frequency range whose wavenumber and intensity increase with increasing Tac and which is sensitive to 16O →18O exchange and disappears on adsorption of water or bases; it can be assigned to surface vibrational modes of alumina.

Dispersion of Adsorbate Vibrational Modes—thec(2×2)Oxygen Overlayer on Ni(100)

The dispersion of surface vibrational modes associated with the $c(2\ifmmode\times\else\texttimes\fi{}2)$ adsorbate layer of oxygen on Ni(100) is measured throughout the two-dimensional Brillouin zone along the [110] direction. Two oxygen modes and two nickel surface phonons are observed. A lattice-dynamical analysis gives good agreement with the data when the oxygen atom sits at \ensuremath{\sim}0.9\AA{} above the nickel surface at the fourfold hollow site.

Atomistic theory of insulator surfaces and interfaces

Surface and interfaces play an important role in many processes including catalysis, corrosion and conductivity. However, they present special problems to their study by either experiment or calculation. Because of this, their properties are generally still poorly understood. This paper describes some recent calculations on the simulation of two-dimensional periodic boundaries where theory and experiment are interacting in a particularly useful way. Surface segregation of impurities or dopants is considered first and the calculations can predict a segregation energy and the Arrhenius behaviour for large crystals. Modification of this behaviour for high surface area materials is also considered. The surface vibrational modes provide a possible way of probing the surface and may give details of defective structures such as over-layers or segregation. The atom-scattering experiments also provide a valuable test of the potentials used in atomistic calculations. Finally, the structure of oxide grain boundaries is considered. In this case, calculation provides a way of interpreting experimental measurements on diffusion or conductivity in terms of atomic processes and boundary structure.

Relation between the condensate fraction and the surface tension of superfluidHe4

The variation of the superfluid order parameter at the liquid surface contributes to the surface tension in proportion to the condensate density. This yields a prediction of the condensate fraction as a function of temperature when proper account is taken of the temperature dependence of the normal components of the surface tension, the largest of which is due to surface modes of vibration. Good agreement is obtained with recent values of the condensate fraction from neutron and x-ray scattering measurements.