R. Schuster

Real-time STM observations of atomic equilibrium fluctuations in an adsorbate system: O/Ru(0001)

Abstract For dynamic studies of surface processes a fast scanning tunneling microscope (STM) was developed, by which imaging rates of up to 20 frames s −1 are achieved. First studies were performed with O/Ru(0001). At 300 K and small coverages the adsorbed oxygen atoms are very mobile, but video sequences recorded with the fast STM allow the random walk of individual oxygen atoms to be monitored. A hopping rate of 14±3s −1 is derived. Longer lifetimes of atom pairs indicate the operation of oxygen-oxygen interactions of the order of kT . At larger coverages these lead to the formation and decay of islands by attachment and detachment of oxygen atoms. Over longer periods of time the atomic configurations change completely, however, without improvement of the order. It is concluded that the system has reached thermodynamic equilibrium and that equilibrium fluctuations are observed. The atomic configurations are reproduced by Monte Carlo simulations with weak attractive interactions between oxygen atoms on (2 × 2) sites.

K-induced restructuring of Au(001) observed by scanning tunneling microscopy

The interaction of the reconstructed Au(001)(5 × 20) surface with adsorbed K atoms was analyzed by means of scanning tunneling microscopy and low-energy electron diffraction (LEED). For K coverages < 0.1 monolayers (ML) the (5 × 20) reconstruction of the substrate is maintained. Higher K coverages lead to the formation of islands of a K-induced reconstruction whose area grows with increasing coverage. The Au(001)(5 × 20) surface reconstruction is completely lifted at 0.25 ML K, whereby in agreement with previous studies a (1 × 2) LEED pattern is observed. From the topography the surface density of Au atoms of the (1 × 2) phase is determined to be 0.5 ML. The (1 × 2) phase is interpreted as a (1 × 2) missing-row substrate structure with the K atoms adsorbed in the reconstruction troughs. Upon high-temperature annealing of the K-covered surface a complex intermixed K/Au(001) surface phase evolves.

Stress-induced recrystallization of a protein crystal by electron irradiation

Ordering of a system of particles into its thermodynamically stable state usually proceeds by thermally activated mass transport of its constituents. Particularly at low temperature, the activation barrier often hinders equilibration—this is what prevents a glass from crystallizing and a pile of sand from flattening under gravity. But if the driving force for mass transport (that is, the excesss energy of the system) is increased, the activation barrier can be overcome and structural changes are initiated. Here we report the reordering of radiation-damaged protein crystals under conditions where transport is initiated by stress rather than by thermal activation. After accumulating a certain density of radiation-induced defects during observation by transmission electron microscopy, the distorted crystal recrystallizes. The reordering is induced by stress caused by the defects at temperatures that are low enough to suppress diffusive mass transport. We propose that this defect-induced reordering might be a general phenomenon.

The system K/Au(111): adsorption and surface restructuring

Abstract The interaction of the reconstructed Au(111) surface with adsorbed K atoms was analyzed by scanning tunneling microscopy and low-energy electron diffraction (LEED). For K coverages between ∼ 0.1 and 0.33 monolayers (ML) the periodicity of the Au(111) Chevron reconstruction decreases with coverage. The K adatoms form a hexagonal (√3 × √3)R30° overlayer on the reconstructed substrate at θ K ≈ 0.33 ML. For higher K coverages islands of a K-induced reconstruction are formed. The Au(111) reconstruction is completely lifted at ∼ 0.5 ML K, whereby a (2 × 2) LEED pattern is observed.

Electronic properties of Cs+CO coadsorbed on the Ru(0001) surface

The variation of the Cs 6s and the Cs 5p emission in He* and Ne* metastable deexcitation spectroscopy (MDS) as a function of the CO exposure indicates a demetallization of the Ru(0001)–(2×2)-Cs and the Ru(0001)–(√3×√3)R30°-Cs surfaces upon CO coadsorption. This observation corroborates a (substrate-mediated) charge transfer from the Cs atom to the 2π* orbital of CO. With the Ru(0001)–(2×2)-Cs system even at CO saturation, MD spectra show emission associated with the Cs 6s state, indicating that the Cs atoms are not completely ionized. Exposing the (√3×√3)R30°-Cs-pre-covered Ru(0001) to CO, surplus Cs of the first layer is displaced into a second layer. In this way, CO molecules are able to be accommodated into the first layer. Desorbing this second layer Cs by heating the sample to 600 K produces a (2×2) structure with one Cs and CO in the unit cell as evidenced by MDS and low energy electron diffraction.

Vibrational Anisotropy of a CO Monolayer on Ni(110)

We have determined the structure of CO/Ni(110) with high accuracy using surface X-ray diffraction. CO molecules are bound in short-bridge sites, tilted alternately to form a lattice with p2mg symmetry. The atomic positions agree well with previous LEED results, but the increased sensitivity permits accurate anisotropic vibrational parameters to be determined as well. We find vibrations which are substantially larger parallel to the surface plane than perpendicular, implying the existence of a low-frequency vibrational mode not seen in an earlier EELS study.

Surface X-ray diffraction on the potassium-induced reconstruction of the Ag(001) surface

Abstract Surface X-ray diffraction experiments have been performed on the K-induced super structures on Ag(001). Both, the (2 × 1) and the (3 × 1) superstructure which are formed at K-coverages of about 0.15 and 0.30 ML, are found to consist of missing row type reconstructions, where one and two Ag-rows are missing along [110¯], respectively. In both cases the K-atoms reside within the large grooves of the missing row structures. For the (2 × 1) structure we find the K-atoms adsorbed on bridge sites relative to second-layer Ag-atoms, thereby coordinated by six Ag-atoms (four first-layer Ag-atoms and two second-layer Ag-atoms) at a distance of 3.44 (5) A. The consideration of anharmonic displacement factors for the top layer Ag atoms is necessary for the proper description of the structure, leading to a significant better agreement with the data as compared to pure harmonic refinement as expressed by the goodness of fit parameter (GOF = 0.93 versus GOF = 1.67). Allowing for anharmonic displacement parameters we determine a Ag interlayer spacing d12 which is contracted relative to the bulk spacing by only 3.2% instead of 12.7% if anharmonic contributions are neglected. In the (3 × 1) superstructure the K-atoms form staggered double rows along [11¯0], in this way forming a distorted quasihexagonal overlayer. The K-atoms are located close to bridge sites to second layer Ag-atoms at a minimum distance of 3.55 (5) A. Additional defects are found in the first Ag-layer. For this more open reconstruction the Ag-interlayer spacing, d12, is contracted by about 10% confirming recent investigations on FCC (110) surfaces regarding the polarization-induced compression of the interlayer distances.

Anomalous coverage behavior of the CsAg distance on CsAg(110)

Abstract The position of Cs on the (1 × 2) missing row reconstructed Ag(110) surface was determined by X-ray diffraction for two different Cs-coverages: θ Cs = 0.2 and θ Cs = 0.3. The Cs was found to be adsorbed in incommensurate chains in the troughs of the missing row with an average adsorption height of 1.7 A ( θ Cs = 0.2) and 1.4 A ( θ Cs = 0.3) above the topmost Ag layer. The apparent contradiction to the classical picture of alkali adsorption, which expects an increase of the Cs adsorption height with coverage, might be partly resolved by introducing a fraction of commensurately adsorbed Cs at θ Cs = 0.2.

Temperature-dependent surface X-ray diffraction on KAg(001)-(2 × 1)

Abstract Temperature-dependent surface X-ray diffraction experiments have been performed on the K Ag (001) -(2 × 1) adsorption system. The structure is characterized by a missing-row geometry in which alternate Ag rows along [110] are missing. The K atoms reside within the large grooves coordinated by six Ag atoms at a distance of 3.44(5) A, corresponding to an effective K-radius of 2.00(5) A. Large anisotropic disorder is observed for both the K-atoms and the top-layer (“ridge”) Ag atoms. The K-atom displacements are largest in the direction along the grooves, whereas for the Ag atoms the vibrations along [110] are significantly larger. The temperature dependence of the Ag vibrations is in accordance with Debye theory for the [110] direction, but deviates from it for the [110] vibrations at high temperature. In contrast to the K-atoms, the out-of-plane vibrations of the top-layer Ag atoms are larger than the in-plane vibrations. The inclusion of anharmonic contributions to describe the Ag disorder significantly improves the fits. It is shown that if anharmonicity is neglected the interlayer contraction is overestimated ( Δd 12 d 12 only −3.2%, instead of −12.7% if anharmonicity is neglected). Due to the anharmonicity, different definitions of the atomic position arise (mean, mode and equilibrium position), which are discussed on the basis of the results.