Karl-Heinz Rieder

Atomic beam diffraction from solid surfaces

Atomic beam techniques are presently being used in many branches of surface physics such as studies of the particle-surface physisorption potential, surface structure, surface phonons, nucleation and growth on metal and insulator surfaces, surface diffusion and accommodation and sticking of molecules. This review concentrates on diffractive phenomena from surfaces, which up to now were investigated mainly with helium. The theoretical background for diffraction calculations is outlined and representative examples of different applications are given. The main subjects covered are: structural determinations of chemisorbed and physisorbed systems, investigations of disordered surfaces, selective adsorption resonances, diffusion and nucleation studies and investigations of growth and phase transitions on surfaces. Diffraction results obtained with Ne, Ar, and are also summarized.

Controlled vertical manipulation of single CO molecules with the scanning tunneling microscope: A route to chemical contrast

A reliable procedure for controlled vertical transfer of single CO molecules between a Cu(111) surface and a scanning tunneling microscope tip and vice versa is demonstrated. It is shown that with a tip having a single CO molecule at its apex, chemical contrast is achieved allowing distinction of adsorbed CO molecules and oxgen atoms, which look very similar to the bare metal tip.

A molecular-beam diffraction study of H2 adsorption on Ni(110)

Abstract The adsorption of H2 on Ni(110) has been studied using He and H2 diffraction. We observe four homogeneous ordered phases as the coverage is increased between θ = 0.7 and θ = 1.6 at a substrate temperature of 100 K. Only two of these phases have been previously observed with LEED. Structural models consistent with the diffraction data are presented for three of these phases. We also observe that the adlayer undergoes on irreversible reconstruction upon heating to temperatures greater than 220 K. Although LEED investigations show a (1 × 2) pattern at high coverages for T > 220 K, our results show that the topmost layer is strongly disordered with a remnant (2 × 6) periodicity. The application of molecular beam techniques to the study of adsorbate covered surfaces is discussed.

Formation of the cyclic ice hexamer via excitation of vibrational molecular modes by the scanning tunneling microscope

Starting from water monomers adsorbed on Cu(111), we have induced the formation of small ice clusters via the tip of a low temperature scanning tunnelling microscope. We show that diffusion, formation, and structural changes of clusters are induced by electrons coupling to vibrational modes of the molecule. The manipulation transfers enough energy into the system to induce stable clusters both two-dimensional and three-dimensional ones, including the important cyclic hexamer. Only the three-dimensional clusters develop a localized gap around the Fermi energy of ≈76 meV, which varies for different positions on a cluster.

Helium diffraction from Ni(100): A study of the clean surface and of hydrogen and carbon adsorption phases

Abstract He scattering from the clean Ni(100) surface gives extremely weak diffraction beams, corresponding to a very small corrugation amplitude of ≅ 0.01 A. Hydrogen adsorption at temperatures between 100 and 200 K leads to the formation of an ordered (1× 1) phase with a corrugation amplitude only three times larger than that of the clean surface. Surface charge-density calculations using overlapping atomic charge densities indicate a normal distance of the hydrogens from the topmost Ni layer dn ≅ 0.9–1.0 A. He diffraction from the c(2 × 2) structure of carbon on Ni(100) confirms the p4g symmetry of this phase, and the best-fit corrugation function reflects the different local distortions of the substrate atoms around the corner and centre carbon atoms. Charge-density calculations yield for the carbon atoms in fourfold hollows dn ≅ 0.1–0.2 A, in good agreement with earlier LEED results.

Snell's Law for Surface Electrons: Refraction of an Electron Gas Imaged in Real Space

On NaCl(100)/Cu(111) an interface state band is observed that descends from the surface-state band of the clean copper surface. This band exhibits a Moiré-pattern-induced one-dimensional band gap, which is accompanied by strong standing-wave patterns, as revealed in low-temperature scanning tunneling microscopy images. At NaCl island step edges, one can directly see the refraction of these standing waves, which obey Snells refraction law.

Site determination and thermally assisted tunneling in homogenous nucleation

A combined low-temperature scanning tunneling microscopy and density functional theory study on the binding and diffusion of copper monomers, dimers, and trimers adsorbed on Cu(111) is presented. Whereas atoms in trimers are found in fcc sites only, monomers as well as atoms in dimers can occupy the fcc as well as the metastable hcp site. In fact the dimer fcc-hcp configuration is only 1.3 meV less favorable with respect to the fcc-fcc configuration. This enables a confined intracell dimer motion, which at temperatures below 5 K is dominated by thermally assisted tunneling.

The coverage-dependent ordering of chemisorbed hydrogen on the (110) surface of nickel

Abstract The dissociative adsorption of hydrogen on Ni(110) at low temperatures has been studied in great detail with He diffraction. With increasing hydrogen exposure, a c(2 × 6), a c(2 × 4), two different c(2 × 6) and a (2 × 1) phase corresponding to coverages of θ = 1 3 , 1 2 , 2 3 , 5 6 and 1 monolayers (ML) have been identified. The corrugation functions of all phases have been determined by intensity analyses of He-diffraction spectra using the hard-wall approximation. The analyses were performed in a model-free manner using symmetry-compatible Fourier representations of the corrugations, and the results were underpinned by additional calculations with the adatoms modeled as Gaussian hills. The best-fit corrugations of all phases up to 1 ML deliver direct pictures of the adatom configurations, and allow an absolute coverage determination on purely crystallographic grounds in good agreement with the relative coverages determined by flash-desorption measurements. The most important structural elements are long H-zig-zag chains along the close-packed Ni rows with the hydrogen atom sites near threefold coordination. The rather long-ranged lateral interaction between the hydrogen chains favors alternation of zig-zag with zig-zag elements in all submonolayer phases. The c(2 × 6) phase with θ = 5 6 ML constitutes an interesting intermediate as 1 6 ML H-adatoms are obviously forced into twofold coordinated sites to form a distorted close-packed hexagonal pattern of H atoms. Thus, the phase jump of 2.5 A of all zig-zag chains necessary to form the (2 × 1) where all close-packed Ni rows are covered with parallel zig-zag chains seems to become feasible via a compression of the c(2 × 6) with θ = 5 6 ML . Further hydrogen can subsequently be accommodated via an adsorbate-induced reconstruction of the Ni substrate. Improved diffraction experiments provide the full two-dimensional corrugation function for the final (1 × 2) structure which corresponds to a saturation coverage of ≅ 1.5 ML whereby probably 1 2 ML hydrogen is adsorbed on the second Ni layer.

Helium diffraction from MgO(100)

Abstract Helium diffraction from the cleavage plane of MgO was studied with different He energies and different angles of incidence. Hard-wall intensity analyses of in-plane and out-of-plane spectra obtained with 63 and 86 meV yielded the corrugation function, which is simply sinusoidal in both unit-cell directions with an amplitude of 0.18 ± 0.02A. A discussion of the total corrugation amplitudes of MgO and NiO on the one side and of LiF and NaCl on the other, in connection with the ionic radii of the respective constituents, provides evidence that the ionic bonding is largely maintained at the surface of the alkali halides, but that an appreciable charge redistribution takes place at the surface of the oxides.

Film growth studies with intrinsic stress measurement: Polycrystalline and epitaxial Ag, Cu, and Au films on mica(001)

Growth and microstructure of thin Ag, Cu, and Au films ultrahigh vacuum deposited onto single‐crystalline mica(001) have been studied in situ by intrinsic stress measurements (ISM) and low‐energy electron diffraction. Depending on the respective substrate temperature, three different modes of Vollmer–Weber (VW) growth can be clearly distinguished by ISM: (i) VW‐type nucleation and subsequent columnar grain growth at low temperatures (110 K) where grain‐boundary relaxation is the prevailing stress contribution, (ii) polycrystalline VW mode in a medium temperature range that is characterized by VW‐type nucleation and grain growth in the continuous film and dominated by the capillarity stress, and (iii) epitaxial VW growth mode at temperatures above 470 K for Ag and 600 K for Cu and Au; here a novel stress mechanism due to the formation of ‘‘single‐crystalline grain boundaries’’ appears during the network stage.