1,079 research outputs found
Structure and stability of a high-coverage (1x1) oxygen phase on Ru(0001)
The formation of chemisorbed O-phases on Ru(0001) by exposure to O_2 at low
pressures is apparently limited to coverages Theta <= 0.5. Using low-energy
electron diffraction and density functional theory we show that this
restriction is caused by kinetic hindering and that a dense O overlayer (Theta
= 1) can be formed with a (1x1) periodicity. The structural and energetic
properties of this new adsorbate phase are analyzed and discussed in view of
attempts to bridge the so-called "pressure gap" in heterogeneous catalysis. It
is argued that the identified system actuates the unusually high rate of
oxidizing reactions at Ru surfaces under high oxygen pressure conditions.Comment: RevTeX, 6 pages, 3 figures, to appear in Phys. Rev. Let
Anisotropic Temperature Factors in the Calculation of Low-Energy Electron Diffraction Intensities
The Adsorption of Atomic Nitrogen on Ru(0001): Geometry and Energetics
The local adsorption geometries of the (2x2)-N and the (sqrt(3)x
sqrt(3))R30^o -N phases on the Ru(0001) surface are determined by analyzing
low-energy electron diffraction (LEED) intensity data. For both phases,
nitrogen occupies the threefold hcp site. The nitrogen sinks deeply into the
top Ru layer resulting in a N-Ru interlayer distance of 1.05 AA and 1.10 AA in
the (2x2) and the (sqrt(3)x sqrt(3))R30^o unit cell, respectively. This result
is attributed to a strong N binding to the Ru surface (Ru--N bond length = 1.93
AA) in both phases as also evidenced by ab-initio calculations which revealed
binding energies of 5.82 eV and 5.59 eV, respectively.Comment: 17 pages, 5 figures. Submitted to Chem. Phys. Lett. (October 10,
1996
Dimer bond geometry in D/Ge(100)-(2×1): A low-energy electron-diffraction structure analysis
The asymmetry of the Ge dimer in the (2×1) reconstruction of Ge(100) is removed upon adsorption of deuterium D. The R-factor analysis indicates a slight remaining asymmetry which is attributed to the coexistence of bare and D-covered dimers. The Ge-Ge bond length of 2.4(2) Å in the dimer does not change within the error limits when compared to the clean surface. The D atoms bond on top of the Ge atoms, exhibiting a Ge-D bond length of 1.6(2) Å
Initial growth of Mg films on Ru(0001): An efficient approximation scheme for the LEED analysis of incommensurate structures
The epitaxial growth of incommensurate Mg layers on a Ru(0001) surface is investigated in the coverage range from submonolayers to 3 ML by analyzing low-energy electron-diffraction LEED I(V) data. For the analysis of the 2-ML Mg film, we developed an efficient approximation scheme that allows the determination of mean interlayer spacings without employing the full unit cell. The Mg-Ru spacing is found to be 2.32±0.05 Å, regardless of the presence of further Mg layers above. The Mg-Mg layer spacing in the Mg bilayer is 5%, expanded with respect to the value of the bulk material, while this layer spacing is expanded by only 2.5% after completion of the third Mg layer. The ABAB... stacking sequence is established from the beginning of the film growth
Oxygen adsorption on the Ru (10 bar 1 0) surface: Anomalous coverage dependence
Oxygen adsorption onto Ru (10 bar 1 0) results in the formation of two
ordered overlayers, i.e. a c(2 times 4)-2O and a (2 times 1)pg-2O phase, which
were analyzed by low-energy electron diffraction (LEED) and density functional
theory (DFT) calculation. In addition, the vibrational properties of these
overlayers were studied by high-resolution electron loss spectroscopy. In both
phases, oxygen occupies the threefold coordinated hcp site along the densely
packed rows on an otherwise unreconstructed surface, i.e. the O atoms are
attached to two atoms in the first Ru layer Ru(1) and to one Ru atom in the
second layer Ru(2), forming zigzag chains along the troughs. While in the
low-coverage c(2 times 4)-O phase, the bond lengths of O to Ru(1) and Ru(2) are
2.08 A and 2.03 A, respectively, corresponding bond lengths in the
high-coverage (2 times 1)-2O phase are 2.01 A and 2.04 A (LEED). Although the
adsorption energy decreases by 220 meV with O coverage (DFT calculations), we
observe experimentally a shortening of the Ru(1)-O bond length with O coverage.
This effect could not be reconciled with the present DFT-GGA calculations. The
nu(Ru-O) stretch mode is found at 67 meV [c(2 times 4)-2O] and 64 meV [(2 times
1)pg-2O].Comment: 10 pages, figures are available as hardcopies on request by mailing
[email protected], submitted to Phys. Rev. B (8. Aug. 97), other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
Density functional study of the adsorption of K on the Ag(111) surface
Full-potential gradient corrected density functional calculations of the
adsorption of potassium on the Ag(111) surface have been performed. The
considered structures are Ag(111) (root 3 x root 3) R30degree-K and Ag(111) (2
x 2)-K. For the lower coverage, fcc, hcp and bridge site; and for the higher
coverage all considered sites are practically degenerate.
Substrate rumpling is most important for the top adsorption site. The bond
length is found to be nearly identical for the two coverages, in agreement with
recent experiments. Results from Mulliken populations, bond lengths, core level
shifts and work functions consistently indicate a small charge transfer from
the potassium atom to the substrate, which is slightly larger for the lower
coverage.Comment: to appear in Phys Rev
Multilayer adsorption and desorption: Cs and Li on Ru(0001)
We use a multilayer lattice gas model for adsorption and desorption to analyze and simulate desorption data for Li and Cs on Ru(0001) extracting surface binding energies and lateral interactions. The latter are repulsive for the first layer and attractive for subsequent ones
Surface x-ray-diffraction study of the Rh(111)+(2×2)−3CO structure
We have studied the geometry of the high-coverage Rh(111)+(2×2)−3CO structure by surface x-ray diffraction. Analysis of the in-plane data set reveals three evenly separated CO molecules per (2×2) unit cell. The evaluation of the crystal truncation rods shows that one CO molecule resides in an on-top site while the other two CO molecules occupy hollow sites. The intensity modulations of the out-of-plane fractional order rods provide geometrical information about distances between the C and O atoms and on the buckling of the CO overlayer
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