Effects of surface relaxation on the surface modes of a CsCl (001) slab shaped crystal

The vibrational frequencies of a 24-layer CsCl (001) slab have been calculated with an eleven-parameter shell model. Without surface relaxation, the results exhibit an instability in the form of a pair of imaginary-frequency Rayleigh modes for wave vectors over most of the surface Brillouin zone. To investigate the effects of relaxation on this dynamic instability, two kinds of relaxation have been used; (i) an inward relaxation of the two outermost planes, and (ii) an expansion of the entire slab in the z-direction (i.e. normal to the surfaces), followed by an inward relaxation of the two outermost planes. In the first case, the imaginary frequencies are removed for most wave vectors except for those very close to the zone center. In the second case, the imaginary frequencies are removed for all wave vectors. Due to the occurrence of two different surfaces of the slab, one containing only Cs⁺ ions, the other only Cl^? ions, two classes of surface modes are found: one with the vibrational amplitudes decreasing from the top layer to the bottom layer, the other vice versa. Longitudinal and transverse optical, and transverse accoustical surface modes are found in both classes. The frequencies of these surface modes are strongly dependent on the surface relaxations.     

Vibrational states of a cobalt dimer on the (111) and (001) copper surfaces

The results of calculations of the total (lateral and vertical) relaxation of the (001) and (111) copper surfaces in the presence of a small cluster of cobalt adatoms, local vibrational density of states and polarizations of these states are presented. The calculations were performed using the atomic interaction potentials in a tight binding approximation. An analysis of the results obtained showed that the presence of a cobalt dimer gives rise to modification of the vibrational states of the copper surface and generation of new modes localized both on the adatoms of the cluster and the surface atoms of the substrate. The revealed anisotropy of surface relaxation along [001] results in deformation of atomic bonding and splitting of the vibrational modes of the dimer. The lifetimes of the vibrational states of the dimer are found to be nearly equal for both surfaces under study, with a frequency shift being however observed. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp.73–78, December, 2008.     

Theoretical study of the surface structure and vibrations of solid argon

The influence of zero-point oscillations on the equilibrium structures and the vibrational modes for the (001), (110) and (111) surfaces of solid Ar is investigated at zero temperature. The surface structures are determined by minimizing the total energy, the static lattice energy plus zero-point energy, where the zero-point energy is evaluated from Einstein frequencies. It is found that the interlayer distances near the surface are larger than the values for bulk argon and are also larger than the distances predicted by minimizing only the static lattice energy. Based upon the surface structures determined by each of these two different minimization schemes, the phonon dispersion curves have been calculated for 21 layer slabs. The qualitative features of the dispersion curves in each case are very similar but there is a significant overall reduction in the frequencies calculated using the structures determined by minimizing the total energy, including the zero-point contribution. The rms vibrational amplitudes of atoms near the surface are calculated from the Einstein frequencies and their values relative to the bulk amplitude are the same as those evaluated by Allen and De Wette [Phys. Rev. 179 (1969) 873] from the lattice dynamics of slabs.     

Surface structure of MgO(001) surface studied by LEED

MgO(001) surfaces prepared by three different heat treatments were studied by LEED. The first was the surface just after the vacuum cleavage; the second was that after the annealing of the first one at 300°C in UHV. The third was prepared by cleaving in air and heating in oxygen until the carbon Auger peak disappear. LEED I-V curves of the diffraction spots, (10), (11) and (20), were recorded and compared carefully. No remarkable difference corresponding to the preparing methods was found. Therefore, the atomic structures of the surfaces seem to be similar to each other. These experimental I-V curves were compared with the theoretical curves obtained by dynamical calculations for several grades of surface relaxation. The theoretical curves with no relaxation or with the smallest one in the calculation (2.5% expand) fitted best with the experimental curves. The experimental curves were also compared with the theoretical ones calculated for the rumpled surface. But no effect induced by the rumpling was found in the experimental curves.     

Irregularities at the {001} surface of MgO: Topography and other aspects

Non-planar sites (e.g. kinks, corners and ledges) are believed to be important in such processes as corrosion, catalysis and crystal growth, but it is difficult to obtain information about their surface relaxation or properties by experimental means. We have therefore studied them using theoretical methods, and present calculations on the topography of non-planar surface sites in the fcc materials MgO, CaO, NiO and NaCl. The models used in the calculations involved both cavities of vacancies within the planar surface and blocks of adatoms above it, and the validity of using such models is discussed. In addition, a more detailed study is undertaken for the doping of non-planar sites in MgO, and comparison is made with the bulk and at the planar {001} surface. Finally, a brief discussion of some thermal excitation energies, both in the planar surface and at selected irregularities in the non-planar surface, is given.     

Cl2 adsorption on MgO(001) surface supported sodium monolayers: a density functional theory study

The adsorption of Cl₂ Na monolayers supported on the MgO(001) surface has been studied by the density functional method using cluster models embedded in a large array of point charges (PCs). The value of PCs was determined by charge self-consistent technique. The results indicate that Na-promoted MgO(001) surface is an efficient catalyst toward Cl₂ adsorptive decomposition. Besides, it was found that the role of the MgO(001) surface is not passive, which is different from CO adsorption on MgO(001) surface supported Na metal monolayers. The analysis of band and projected density of states indicates that the electron transfer from the surface Mg 3s valence orbital and Na 3s valence orbital to the anti-bonding σ^∗ orbital of Cl₂ is the source of the Cl₂ bond weakening. This is also different from the CO adsorption on MgO(001) surface supported Na metal monolayers, where only the electrons from the Na valence orbital are transferred to the anti-bonding π^∗ orbital of adsorbed CO. Our study suggests that the essence of catalysis is different for CO and Cl₂ adsorption on Na metal monolayers supported an MgO(001) surface.     

Adsorption of <Emphasis Type="Italic">d</Emphasis>-metal atoms on the regular MgO(001) surface: Density functional study of cluster models embedded in an elastic polarizable environment

Structure and bonding of complexes formed by 17 different single transition metal atoms (Cu, Ag, Au; Ni, Pd, Pt; Co, Rh, Ir; Fe, Ru, Os; Mn, Re; Cr, Mo, W) with oxygen sites of the regular MgO(001) surface were studied computationally. We employed an all-electron scalar-relativistic density functional method in combination with our novel scheme of cluster models embedded in an elastic polarizable environment that allows one to account for substrate relaxation. Even on a rigid substrate such as ideal MgO(001), adsorbate-induced relaxation noticeably affects structure and stability of surface complexes. For more reliable estimates, we calculated adsorption energies with two different gradient-corrected exchange-correlation functionals, BP86 and PBEN. More than one electron configuration was considered for metal atoms exhibiting high-spin states adsorption complexes. Within one group of the periodic table, 3d-atoms, in general, were found to adsorb more strongly than 4d-atoms, but weaker than 5d-atoms. In line with our previous studies of selected d-metal atoms adsorbed on oxides, the surface complexes considered did now show any indication of metal oxidation. PACS 68.43.Bc; 68.43.Fg; 71.15.Nc; 82.65.+r; 68.35.Ct; 68.43.-h; 73.20.Hb; 71.15.Mb; 75.70.Cn     

Theoretical modelling of surface phonons

We present a mini review of progress made towards theoretical modelling of surface phonons. We outline the essential ingredients of two theoretical methods, viz. an adiabatic bond charge method for semiconductor surfaces and the ab-initio density-functional perturbation method for solid surfaces in general. From the results of theoretical calculations we establish trends and criteria for the existence of localized phonon modes on group-IV(001) and III-V(110) semiconductor surfaces. We further obtain signatures of characteristic vibrational modes which develop during dissociative molecular adsorption on Si(001) surfaces. The results are compared with available experimental measurements. Some remarks are forwarded regarding manipulation of surface phonon modes for scientific advances and technological applications.      

Effects of O defects on adsorption of small Ag clusterson a MgO(001) surface

The interaction of Ag atoms with a defective MgO(001) surface is systematically studied based on density functional theory. The Ag clusters are deposited on neutral and charged oxygen vacancies of the MgO(001) surface. The structures of Ag clusters take the shape of simple models of two- or three-dimensional (2D and 3D) metal particles deposited on the MgO surface. When the nucleation of the metal clusters occurs in the F_s (missing neutral O) centre, the interaction with the substrate is considerably stronger than that in the F_s⁺ (missing O^- ) centre. The results show that the adsorption of Ag atoms on the MgO surface with oxygen vacancy is stronger than on a clear MgO surface, thereby attracting more Ag atoms to cluster together, and forming atomic islands.     

AB initio hartree-fock study of the MgO(001) surface

The MgO(001) surface has been studied with an ab initio Hartree-Fock crystalline orbital LCAO program. An optimized basis set containing nine atomic orbitals (three s and six p) per atom has been used. The semi-infinite crystal has been simulated by a slab containing three planes (six atoms per cell). In agreement with the most recent LEED experiments, no relaxation is found and the “rumpling” is very small (1% of the nearest neighbour separation in the bulk). The analysis of the electron charge distribution and of the ion multipoles shows that the fully ionic character found for the bulk is maintained at the surface, and that the anion deformation is very small. A surface energy of 1.43 J/m² was obtained.     

Charging effects on the vibrational properties of Au and Au2 on MgO(100)

Vibrational spectra of charge-neutral and charged Au and Au₂ on MgO(100) were investigated using ab-initio density functional perturbation theory. The calculated vibrational spectra showed vibrational features associated with the charge states of Au and Au₂ on MgO(100). Further analyses of surface in-plane and normal phonon modes of Au and Au₂ on MgO(100) were performed to extract vibrational features involving the Au modes. These features provide important information for experimentally explaining the charge states of Au and Au₂ on MgO(100).     

Structure and phonon spectrum of a submonolayer Ni film on the surface of Cu(100)

The equilibrium atomic structure and the phonon spectra of a submonolayer (θ = 0.5 monolayer) Ni film deposited on the surface of Cu(100) are calculated using the potentials obtained by the embedded atom method. We consider atomic relaxation, the vibrational state density distribution on Ni and substrate atoms, and polarization of vibrational modes. Variation of the phonon spectrum upon segregation of Cu atoms on the film surface is considered. It is shown that mixing of vibrations of Ni adatoms with vibrations of substrate atoms occurs in the entire frequency range, leading to a frequency shift of the vibrational modes of the substrate and to the occurrence of new vibrational states atypical of a clean surface. The Cu(100)–c(2 × 2)–Ni structure is dynamically stabler when placed in the subsurface layer of the substrate.     

Surface relaxation and thermal expansion for the (001) face of α-Fe and Cu

The surface relaxation and the near-surface enhancement of thermal expansion have been calculated for the (001) face of a bcc crystal, α-Fe, and an fcc crystal, Cu. The calculations make use of the anharmonic perturbation formalism of Dobrzynski and Maradudin; the results for certain equal-time vibrational correlation functions which arise in this formalism are also presented. The crystal potential is described in terms of several kinds of short-range empirical interatomic potentials, such as have been used in studies of defects in bulk; in the near-surface region, the effects of surface redistribution of the electron distribution are modelled by the addition of a simple surface Madelung (SSM) force. The effect of the SSM force is to limit severely the usual outward relaxation driven by short-range interatomic potentials. For Fe(001), the five and one-half percent outward static relaxation driven by the short-range potentials acting alone is changed to a one percent inward static relaxation when the SSM force is incorporated; for Cu(001), the comparable change is from a one percent outward relaxation to a one-half percent outward relaxation. On the other hand, the SSM force makes only a small effect on the surface-enhanced thermal expansion coefficients (STEC) for interplanar spacings. The STEC for the outermost spacing is between 2.5. and 3.0 times of that for the bulk at the Debye temperature for both Fe(001) and Cu(001); for the second interplanar spacing, the STEC is smaller than 1.5 times of that of the bulk at the Debye temperature. The ratios of the near-surface mean-square amplitudes (MSA) to those of the bulk at high temperatures are, for Fe(001), about 1.75 for z-components (normal to surface) and 1.55 for x-components (parallel to surface) in the surface layer; for Cu(001), about 1.95 for z-components and 1.30 for x-components. The interplanar correlation functions, while smaller than the MSA on an absolute scale, do show considerable surface-enhancement, particularly for the zz-compoments. For example, the zz-correlation between an atom in the outermost layer and its nearest neighbor in the next layer is nearly twice the comparable bulk correlation above the Debye temperature for both Fe(001) and Cu(001).     

Dynamical model of selective versus nonselective laser-stimulated surface processes

The selective and nonselective excitations of laser-stimulated surface processes are quantitatively discussed by the population dynamics of multi-photon-multi-phonon processes. The multi-level system is reduced to a few-level system in the Heisenberg-Markovian picture and the level populations of different normal modes of the adspecies/surface system are solved numerically. It is shown that specific normal modes of the system may be selectively excited without appreciably heating the system as a whole when the pumping rate is faster than the energy relaxation rate. In contrast to a homogeneous gas-phase system (governed by picosecond phenomena), much longer time scales of the relaxation dynamics are discussed for the present heterogeneous system.     

On the mechanism responsible of Raman enhancement on carbon allotropes surfaces: the role of molecule‐surface vibrational coupling in SERS

The ability of different carbon allotropes surfaces as potential substrates for enhanced Raman spectroscopy is analysed theoretically and the factors responsible of the Raman‐enhancing mechanism deeply scrutinised. Our analysis is based on the partition of the Raman tensor into molecule and surface terms, which leads to three different contributions to the Raman activity (‘molecule’, ‘surface’ and ‘intermolecular’). Both static and pre‐ resonance conditions are considered in our analysis of the Raman spectra of pyridine adsorbed on model planar and curved surfaces and the three contributions to the Raman activity obtained separately. At static conditions, there is a general decrease in the Raman activity of vibrational modes associated to the molecule, proportional to the strength of the molecule–surface interaction. This stems from a reduction of the polarizability of pyridine upon its adsorption on the carbon surface. Under pre‐resonance conditions, the surface contributes significantly to the Raman activity of the pyridine vibrational modes, even if the electronic transition involves exclusively energy levels from the surface. This is because of small vibrational couplings between molecular and surface modes which are negligible in metallic surface‐like silver. It suggests also the possibility of finding similar effects in metallic surface built from lighter atoms like silicon or aluminium. Copyright © 2015 John Wiley & Sons, Ltd.     

Characterization of epitaxial MgO growth on Si(001) surface

MgO epitaxial growth on a Si(001) surface by ultrahigh-vacuum molecular beam epitaxy was investigated. Epitaxial orientation and crystalline quality were characterized based on the three-dimensional reciprocal map obtained by Weissenberg RHEED. The epitaxial orientation and crystallinity were strongly dependent on the initial condition of the substrate. When MgO was deposited on a clean Si(001) surface at room temperature a MgO(001) film grew on the Si(001) substrate with two in-plane orientations:MgO[110]//Si[100] and MgO[100]//Si[100]. This is the first observation of MgO epitaxy with the former orientation, which has a smaller mismatch than the latter orientation. When the substrate was exposed to O₂ or thermally oxidized, the latterorientation predominantly grew on the substrate. Deposition of Mg on the substrate also produced the latter orientation. These results imply that nucleation sites on the initial substrate play an important role in determining the epitaxial orientation.     

The spectroscopy of surface vibrations by atom scattering

The recent progress in the production of highly monochromatic atomic beams is opening new perspectives in surface physics, having paved the way for a full determination of the surface vibrational structure. After a discussion on the possible determination of Rayleigh wave dispersion curves from angular distributions exploiting the kinematical focussing effect, a short review is presented on the direct measurement of surface phonon dispersion curves, first achieved by Brusdeylins, Doak and Toennies in alkali halides, from time-of-flight (TOF) spectra of scattered He atoms. A comparison is made with the existing theories of surface phonons in ionic crystals. The state of the art in the theory of inelastic processes is briefly illustrated in order to discuss the theoretical interpretation of TOF spectra. The one-phonon energy loss spectra of He scattering from LiF(001) calculated for a hard corrugated surface model are found to be in general good agreement with the experimental TOF spectra. From such a comparison evidence is obtained that: i) one-phonon processes are predominant, and ii) in addition to Rayleigh waves important contributions to the inelastic scattering come from the surface-projected density of bulk phonons. Important effects due to inelastic resonances with surface bound states are put in evidence and explained by simple kinematical arguments. The possible observation of surface optical modes in NaF(001) is finally discussed.     

Effect of disorder on ultrafast exciton dynamics probed by single molecule spectroscopy

We present a single-molecule study unraveling the effect of static disorder on the vibrational-assisted ultrafast exciton dynamics in multichromophoric systems. For every single complex, we probe the initial exciton relaxation process by an ultrafast pump-probe approach and the coupling to vibrational modes by emission spectra, while fluorescence lifetime analysis measures the amount of static disorder. Exploiting the wide range of disorder found from complex to complex, we demonstrate that static disorder accelerates the dephasing and energy relaxation rate of the exciton.     

The high temperature vibrational entropy of (001) surfaces of ionic crystals—Effects of relaxation and rumpling

The high temperature vibrational surface entropy of (001) alkali halide surfaces expressed in terms of entropy per surface atom is much smaller than for the same metal surface if relaxation is neglected. Plausible relaxation and rumpling tends to reduce the surface entropies still further.     

Intramolecular vibrational dynamical barrier due to extremely irrational couplings

The intramolecular vibrational dynamics due to extremely irrational couplings is demonstrated by contrast to the resonance couplings, for the three-mode case of H分子物理学 分子内振动弛豫 甚高频无理耦合 共鸣 振动动力学intramolecular vibrational relaxation, extremely irrational couplings, resonanceProject supported by the National Natural Science Foundation of China (Grant No~20373030) and the Foundation for Key Program of Ministry of Education, China (Grant No~306020) and the Specialized Research Fund for the Doctoral Program of Higher Educatio2007-03-04The intramolecular vibrational dynamics due to extremely irrational couplings is demonstrated by contrast to the resonance couplings, for the three-mode case of H2O as an example. The extremely irrational couplings are shown to impose such strong hindrance to intramolecular vibrational relaxation （IVR） that they act as barriers. They restrict the direct action/energy transfer between the two stretching modes, though they allow the transfer between a stretching and a bending modes. In contrast, the resonance is more mediated by the bending mode and leads to chaotic IVR. It is also shown that there is a region in the dynamical space in which resonance and extremely irrational couplings coexist.