Perturbation calculations of the c(2×2) sodium overlayer structure on Al(001)

The chemisorption bond-length and binding location of the c(2×2) sodium overlayer on Al(001) surface are determined by a convergent perturbation calculation of the dynamical LEED approach. The use of convergent perturbation calculation allows accurate surface structures to be determined with very economical amounts of computation effort. Efficiency features specific to our computation method are outlined. Using a set of simple dynamical inputs, we determine that the sodium atom sits in a 4-fold coordinated binding site with a NaAl chemisorption bond-length of 2.08 ± 0.12 Å.     

Force-field calculations of the packing energy of monolayers of C3 and C4 hydrocarbon molecules adsorbed on single crystal metal surfaces

Force-field calculations are applied to investigate the packing energy of overlayers of C₃ and C₄ hydrocarbons adsorbed on Pt(111) and Rh(111), in which Van der Waals adsorbate-adsorbate interactions play a significant role. The packing energies are used to compare various adsorbate structures for which Low Energy Electron Diffraction patterns have been obtained and corresponding detailed structures have been proposed. In particular, the conformation of methylacetylene (HCCCH₃) in a (2 × 2) arrangement, and of propylidyne (CCH₂CH₃) and butylidyne (CCH₂CH₂CH₃) in (2 × 2), c(4 × 2) and ($\text{2}\text{3}\text{× 2}\text{3}\text{)R30°}$ arrangements are considered.     

Adsorbate-adsorbate interactions and the ordering of organic monolayers on metal surfaces

To study the ordering of molecules adsorbed on single-crystal substrates, a molecular cross-section (MCS) is defined, which measures the surface area occupied by each molecule. With this MCS, a two-dimensional packing coefficient C_2D is then defined for ordered arrays of adsorbed molecules. Values and trends for MCS and C_2D are discussed for known surface structures, especially for benzene adsorbed on metal surfaces. The packing is found to be generally less dense at surfaces than one would expect from comparison with packing in three-dimensional organic crystals. The Van der Waals packing energy and the repulsive dipole-dipole energy are also computed to study this issue. The lack of close-packing is attributed to the need to respect structural coincidence with the substrate and/or co-adsorption of small molecules like CO. These concepts are then applied to the prediction of the long-range order that a monolayer of adsorbed molecules may adopt: thereby possible adsorption structures can be defined, restricting the number of possibilities in a further structural determination.     

The structure of the c(2 × 2) oxygen overlayer on the unreconstructed (110) surface of iridium

An Ir(110)-c(2 × 2)O structure has been prepared by adsorbing a half-monolayer of oxygen at room temperature on an unreconstructed (1 × 1)Ir surface stabilized by a quarter-monolayer of randomly adsorbed oxygen. Results of the low energy electron diffraction structural analysis indicate that the ordered oxygen atoms are residing on the short-bridged sites on the (110) surface. The Ir-O interlayer spacing is 1.37 ± 0.05 Å, and the bond length is 1.93 ± 0.07 Å. The topmost substrate interlayer spacing is found to be 1.33 ± 0.07 Å rather than 1.26 ± 0.07 Å which is the topmost interlayer spacing of the unreconstructed (1× 1)Ir surface.     

The surface reconstructions of the (100) crystal faces of iridium,platinum and gold: II. Structural determination by LEED intensity analysis

The investigation, in a companion paper, of the reconstructions of the Ir(100), Pt(100), and Au(100) crystal surfaces is completed here with an extensive analysis of low energy electron diffraction (LEED) intensities, using dynamical (multiple scattering) calculations. It is found that a hexagonal rearrangement of the top monolayer is a likely explanation of the surface reconstruction. At least for Ir and Pt (no calculations were made for Au), this hexagonal layer would have a registry involving bridge sites on the next square unit cell metal layer and it is contracted and buckled. Bond length contractions parallel and perpendicular to the surface occur; the Pt top layer is rotated by a small angle (0.7°) with respect to the substrate. A second model that cannot be ruled out by the LEED analysis, but disagrees with ion-scattering data, involves shifted close-packed rows of top-layer atoms and requires domain structures in the case of Pt and Au. Charge-density-wave and missing-row models are ruled out by our structure analysis. A correlation is found between the occurrence of surface reconstructions on metals and a small ratio of their Debye temperature to their melting point. This correlation singles out mainly the 5d metals as having a propensity to surface reconstruction. The effects of adsorbates on the reconstructions are also discussed.     

Fast ion beam collinear laser spectroscopy: Some aspects of recent applications,sensitivity and resolution

This contribution deals with some selected aspects of Fast Ion Beam Collinear Laser Spectroscopy: (1) the study of h.f.i. in free ions; (2) sensitive state selective particle detection; (3) laser-RF double resonance; and (4) laser frequency Doppler tuning in external magnetic fields.     

Factorization in high energy hadron collisions

The concept of factorization is discussed for elastic diffraction scattering and diffraction dissociation of hadrons at high energy. In addition to the usual definition in terms of the t-channel, a natural definition of factorization in the s-channel is proposed and compared with the former. It is shown that s-channel factorization of all diffractive processes is consistent with the assumption that elastic scattering is identical to the shadow of the diffraction dissociation processes.     

Surface structures of cubo-octahedral Pt-Mo catalyst nanoparticles from Monte Carlo simulations

The surface structures of cubo-octahedral Pt-Mo nanoparticles have been investigated using the Monte Carlo method and modified embedded atom method potentials that we developed for Pt-Mo alloys. The cubo-octahedral Pt-Mo nanoparticles are constructed with disordered fcc configurations, with sizes from 2.5 to 5.0 nm, and with Pt concentrations from 60 to 90 atom %. The equilibrium Pt-Mo nanoparticle configurations were generated through Monte Carlo simulations allowing both atomic displacements and element exchanges at 600 K. We predict that the Pt atoms weakly segregate to the surfaces of such nanoparticles. The Pt concentrations in the surface are calculated to be 5-14 atom % higher than the Pt concentrations of the nanoparticles. Moreover, the Pt atoms preferentially segregate to the facet sites of the surface, while the Pt and Mo atoms tend to alternate along the edges and vertexes of these nanoparticles. We found that decreasing the size or increasing the Pt concentration leads to higher Pt concentrations but fewer Pt-Mo pairs in the Pt-Mo nanoparticle surfaces.     

Circular dichroism in K-shell ionization from fixed-in-space CO and N2 molecules

We have measured the angular distributions of 1s photoelectrons excited by circularly and linearly polarized light from fixed-in-space CO and N2 molecules, in the vicinity of their shape resonances. A strong circular dichroism, i.e., a strong dependence on the sense of rotation of the polarization vector of the photons, is found for both molecules. State-of-the-art one-electron multiple scattering and partially correlated random phase approximation calculations are in good agreement with many, but not all, aspects of the experimental data.     

Z4-Kerdock Codes, Orthogonal Spreads, and Extremal Euclidean Line-Sets

When m is odd, spreads in an orthogonal vector space of type⁺(2m + 2,2) are related to binary Kerdock codes and extremalline-sets in 2^{m + 1} with prescribed angles. Spreads in a 2m-dimensionalbinary symplectic vector space are related to Kerdock codesover Z₄ and extremal line-sets in with prescribed angles. These connections involve binary, realand complex geometry associated with extraspecial 2-groups.A geometric map from symplectic to orthogonal spreads is shownto induce the Gray map from a corresponding Z₄-Kerdock codeto its binary image. These geometric considerations lead tothe construction, for any odd composite m, of large numbersof Z₄-Kerdock codes. They also produce new Z₄-linear Kerdockand Preparata codes. 1991 Mathematics Subject Classification:primary 94B60; secondary 51M15, 20C99.