Substrate strain induced interaction of adatoms on W (110)

The interaction of adatoms due to elastic strains created in an elastically isotropic substrate is investigated. For cases where the adatoms occupy sites with low symmetry, an angular dependent interaction results which falls off as s^–3 at large distances. An exact expression is given for the long range interaction in terms of an anisotropy parameter of the force dipole tensor. The short range interaction is calculated by introducing a smooth cutoff. Interactions of adatoms on near neighbour sites on W (110) are given.     

Surface atom displacement processes

Progress in field ion microscope studies of adatom displacements on metal surfaces is reviewed. It is concluded that of the displacement processes that contribute to surface diffusion only displacements between low-coordination (terrace) sites are well characterised. Procedures and preliminary results of FIM studies of adatom displacement over steps are described. Activation energies measured for passage of Ta, W, Re, Ir and Pt adatoms across (110) W steps are found to equal activation energies for diffusion over (110) W, despite the highly reflecting character of the step for all the adsorbates except Pt. Displacements of adatoms interacting with other adatoms are discussed. Results presented show that interaction of transition metal adatoms forming close-packed dimers on (110) W is rather weak, with a minimum interaction energy [?U(r) < 4kJ/mol] for Re₂ corresponding to a very weak attraction for Re adatoms 0.27 nm apart.     

A re-interpretation of the LEED structures formed by iodine on W(l10)

The early work done by Avery on the adsorption of I₂ on W(110) has been re-interpreted using adatom models originally developed for the Cl₂/Br₂/I₂/Fe (100) systems. The experimental coverages and LEED patterns are described precisely using variable, non-coincident nets of halogen atoms. It is shown that the movement of spots within the diffraction pattern arises from the movements of iodine atoms along simple crystallographic directions. The model assumes repulsive lateral interactions between iodine adatoms which is consistent with the desorption behaviour. The reasons for structural changes within the adlayer are discussed using the model, and the internuclear spacings and geometry of the adlayer are shown to be consistent with previous work on Fe(100) and W(100).     

A calculation of surface diffusion coefficients of adsorbates on the (110) plane of tungsten

An attempt to calculate the prefactors of the diffusion coefficients of tungsten, xenon and oxygen atoms on the W(110) plane in the zero-coverage limit was made, using a Fokker-Planck formalism. Pairwise substrate-substrate and adsorbate-substrate atom interactions were assumed and expressed by appropriate Morse potentials. The vibrations of the substrate were approximated by an Einstein model of independent oscillators. When Morse potentials were so adjusted as to give good agreement between experimental and calculated activation energies, a prefactor in excellent agreement with experiment was calculated for W/W(110). For Xe/W(110) and O/W(110) the calculated values were ～10^?4 cm²/s, i.e. essentially “normal”, while the experimental values are much lower. Possible reasons for the discrepancies are discussed.     

The role of F A1:Ag+ defects in laser light generation and coadsorption of CO and halogen atoms at the KCl and KBr surface sites. First principles calculations

F _A1:Ag⁺ color center at the low coordination (100) and (110) surface sites of KCl and KBr thin films play an important role in providing tunable laser oscillation and adsorbatesubstrate interactions. Double-well potentials at this site are investigated using ab initio molecular electronic structure calculations. The calculated Stokes shifted (optical transition bands), opticaloptical conversion efficiencies, the probability of orientational destruction, exciton (energy) transfer and Glasner-Tompkins empirical rule suggest that laser light generation is sensitive to (i) the lattice anion, (ii) the coordination number of surface ions, and (iii) the choice of the basis set centered on the anion vacancy. The adsorbate-substrate interactions were found to be dependent on the electronegativity of the adatom and on the lattice anion. Optimised geometries and the coadsorption of CO and (F, Cl, Br, I) on KCl and KBr (100) crystals are presented. Calculated chemisorption energies for CO on the (halogen atom/defect free sites of KCl and KBr (100) crystals) showed that the coadsorption of halogen atom tends to block other adsorbate-substrate interactions at the nearest neighbour sites. Thus if halogen atom coverage increases, the CO prefers to be adsorbed on the K⁺ site of the KCl and KBr (100) surfaces and on KBr relative to KCl.      

An estimation of surface vibration excitation cross sections of CO,O and H adsorbed on tungsten

A simple model is described for the evaluation of adatom loss spectra of dipole vibrations perpendicular to the substrate. It considers the inelastic electron scattering before and after specular reflection on the substrate. By comparing the loss to elastic peak area the total cross section σ of the inelastic electron scattering can be estimated. The model was used for evaluating experimental results published in the literature by Ibach, Froitzheim, Adnot, Backx and Barnes on the systems WCO, WO and WH. The main results are: for the CO 258 meV loss peak, σ = (11.6?18.3)×10^?18cm²; for the O 78 meV loss peak, σ = (8.4?16.2) × 10^?18cm²; and for the H 130 and 155 meV loss peaks, σ =(0.73?2.2) × 10^?18cm². They are close to the theoretical values. A reasonable agreement was found between σ values determined on WO and WβCO (dissociated) systems.     

A new cluster model interpretation of the anomalous lifetime of <Superscript>14</Superscript>C

A new cluster model solution to the long-standing nuclear structure problem of describing the anomalously long lifetime of ¹⁴C is presented. Related beta-decay data for ¹⁴O to states in ¹⁴N, gamma-decay data between low-lying positive parity states in ¹⁴N and the elastic and inelastic magnetic dipole electron scattering from ¹⁴N data are all shown to be very accurately described by the model. The shapes of the beta spectra for the A = 14 system are also well reproduced by the model. The model invokes four-nucleon tetrahedral symmetric spatial correlations arising from three- and four-nucleon interactions, which yields a high degree of SU(4) singlet structure for the clusters and a tetrahedral intrinsic shape for the doubly magic ¹⁶O ground state. The large quadrupole moment of the ¹⁴N ground state is obtained here for the first time and arises because of the almost 100% d-wave deuteron-like-hole cluster structure inherent in the model.     

Direct observation of adatom-substitutional atom interaction on dilute metal alloy surfaces

The field ion microscope is used to study interactions of a migrating tungsten adatom with substitutional rhenium atoms in the (110) plane of a W-3%Re alloy. By observing about 300 migration periods on a single (110) surface plane of a small field ion emittier, each position of the adatom with respect to deduced Re atom locations can be identified. The interaction is found to be attractive with a strength of 90 ± 7 meV at the closest equilibrium separation, and repulsive with a strength of at least 80 meV at the second closest separation. No interaction could be observed for larger separations indicating a strength of less than 10 meV. Results of a control experiment, diffusion of single W adatoms on the (110) plane of pure W, are also presented for comparison.     

Surface resonant states and the adsorption of 4d atoms on Mo (110) and (100)

The adsorptive properties of Mo (110) and Mo (100) relative to 4d transition adatoms are investigated and compared using a tight-binding model. The surface resonant states existing on the (100) free surface are shown to have a drastic influence. On Mo (110) a Mo adatom retains a strong atomic character while on Mo (100) one approaches a surface molecule limit which is due to a strong interaction between the adsorbate atomic state and the free surface resonance. Consequently a large anisotropy of binding energy is found. The case of different 4d adatoms is also discussed.     

Phase diagram of the fcc(1 1 0) surfaces with adsorbate-induced missing-row reconstructions

The phase diagram of the fcc(1 1 0) surfaces with missing-row reconstructions induced by adatoms, is calculated by use of the Blume–Emmery–Griffiths model. In the model, we introduce adatom–adatom interactions to determine surface structures and dipole–dipole interactions to describe the effect of zigzag adsorption. The interactions between nearest-neighbor (NN) and next-nearest-neighbor (NNN) rows are considered. The calculation of the temperature versus adatom chemical potential phase diagram is performed using mean-field approximation. It is indicated that if NN and NNN interactions are competitive, there appear either dipole or coverage modulated (incommensurate) phases at high temperatures for a wide range of the interactions.     

The three-adatom non-pairwise (“trio”) interaction,with implications for Monte Carlo simulations of O/W(110)

Ching et al.¹ found that three-adatom non-pairwise interactions, with a repulsive energy greater than a quarter the nearest pair attraction, can explain the pronounced asymmetries in the phase diagram of O/W(110). In the first explicit calculations of such short-range “trio” interactions, we find that electronic indirect interactions have strengths consistent with their parameters. In general the closest-spaced trios are stronger than all but the few shortest-range pair interactions which determine the adlayer symmetry. We further show that triad configurations with similar shorter legs should have comparable magnitude; when applied to O/W(110), this idea doubles the number of trios entering the equations of Ching et al.¹ and thus can halve the minimum required trio strength. Such trio interactions (as well as linear triad configurations) have a substantial quantitative effect on the pair interaction strengths extracted from Monte Carlo simulations.     

Electronic structure and adsorption properties of the system Cs/O/W(110)

Comparative studies are carried out of the Cs/O/W(110) and Cs/W(110) adsorption systems. The method of threshold photoemission spectroscopy is used to study the work function and electronic structure in the energy region near the Fermi level as functions of the sub-monolayer cesium coverage. A significant increase of the saturation cesium coverage is observed on the O/W(110) surface. A new adsorption-induced surface band is observed in the electronic spectrum of the system Cs/O/W(110) with a binding energy ∼0.7 eV. For coverages of about one monolayer metallization of the adsorbed layer is observed. It is shown that the electronic structures of the systems Cs/O/W(110) and Cs/W(110) are similar for low coverages. A difference in the adsorption properties for these two systems occurs for coverages close to one monolayer, which is explained by the creation of new interaction centers of the Cs adatoms on the W(110) surface in the presence of oxygen. Fiz. Tverd. Tela (St. Petersburg) 39, 1683–1686 (September 1997)     

The specific oxidation mechanism in the initial stages of O2 adsorption on submonolayer Ba covered W(1 1 0) surface

The initial oxidation of a polarized 0.35-ML Ba-covered W(1 1 0) surface has been investigated in detail by high-resolution photoemission spectroscopy using synchrotron radiation. Upon small exposures (?0.15 L) to O₂, both interfacial W 4f (Ba-coordinated W) and Ba 4d core levels shift simultaneously toward smaller binding energies and then fix during 0.15-0.25 L, finally disappear at 0.6 L. The concurrent negative shifts of both interfacial W 4f and Ba 4d peaks can be attributed to be a consequence of the increased occupancy of 5d states. This explanation is supported by the variations of 5d stats in valence band spectra upon the initial oxidation. Our results clearly demonstrate that charge rearrangement is the dominant effect in the surface reactivity of our oxidized surface. We also found that this charge redistribution is associated with a structural change during oxidation, which is in contrary to previous studies of alkali-metal promotion. That is, oxygen is first chemisorbed atop the polarized Ba adlayer, then incorporates beneath Ba adatoms to form a covalent Ba-O-W complex upon further dosage, and reacts strongly with the Ba overlayer to become an ionic Ba⁺²O⁻² surface dipole layer at 0.6 L.     

Etude par diffraction des neutrons de la structure magnetique de l'antiferromagnetique La2Fe2S5

The magnetic structure of orthorhombic La₂Fe₂S₅ is found to be monoclinic with space group P₂. It points out that each kind of magnetic ions (octahedral or tetrahedral) which are separetely found on chains parallel to [001] are antiferromagnetically coupled on each chain through super- exchange interactions. Nearest neighbour antiferromagnetic chains are uncoupled; coupling between next-nearest neighbour chains is realized through super-superexchange interactions of the type Fe-S-S-Fe or possible more complex paths including La³⁺.     

Measurement of the barrier height of the reflective W {110} plane boundaries in surface diffusion of single atoms

Plane boundaries are in general reflective to diffusing atoms. From about 2000 heating periods of direct field ion microscope observations we have studied adatom plane edge interaction and determined the barrier heights of the W {110} plane boundaries in single atom diffusion of W, Re, and Ir. Using equations we have derived recently, the barrier heights ΔE_b are found to be about 200 meV for these adatoms.     

Ab initio calculations of multipole moments,polarizabilities and long-range interaction coefficients for the azabenzene molecules

Using LCAO-SCF wave functions on the monomers and a non-empirical Unsöld procedure for the second-order properties we have calculated the (2 ^l ) multipole moments (up to l=6), the (l,l') multipole polarizabilities (up to l + l' = 6) and the related long-range coefficients describing the electrostatic, induction and dispersion interactions for the different azabenzene molecules. The agreement with available experimental data is good, in particular for the dipole polarizabilities. The anisotropy of the long-range interaction potential is dominated by the electrostatic contributions, although the dispersion terms, especially the mixed-pole terms (l≠l') for even n (C₈, C₁₀), also contribute significantly; the induction energy is rather small. The π contributions to the polarizabilities and the dispersion interactions are found to be larger than earlier estimates. Moreover, it is shown by calculating the dipole polarizabilities of some (aza)naphthalenes and (aza)anthracenes, that a bond polarizability model can be applied effectively only if the delocalized π electrons are considered separately from the σ electrons.     

Elestic constants in the interfacial layer at the nematic-liquid-crystal-vapour interface

Summary Close to the interface between a nematic liquid crystal (NLC) and another medium, the elastic constants become functions of distancez from the interface and of angle θ between the directorn and the unit vectork orthogonal to the interface. Furthermore, due to the breaking of the translation symmetry at the interface, a lot of new subsurface elastic contributions can appear. In a previous paper we investigated these subsurface anomalies by using a simple molecular model based on induced-dipole-induced-dipole interactions and by making numerical calculations in the special case of a planar director distortion. In this way, only the numerical values of some effective subsurface elastic constants that characterise planar director distortions could be obtained. In this paper we make a more complete analytical calculation of all the subsurface elastic constants by using a microscopic model and a more general theoretical procedure. The microscopic interaction energy is written in a general form that allows us to investigate different kinds of intermolecular interactions (induced diple-induced dipole, quadrupole-quadrupole and so on). Both thez-dependence and the θ-dependence of the subsurface elastic constants are obtained in a closed analytical form. In the special case of induced-dipole-induced-dipole interactions and for planar director distortions, our analytical results are shown to agree with the previous numerical results. The important macroscopic effect of these elastic subsurface anomalies is discussed.