A refined LEED structural determination for the surface designated Ni (111)–(2 × 2)–S

1989 ◽  
Vol 67 (11) ◽  
pp. 1975-1979 ◽  
Author(s):  
Y. K. Wu ◽  
K. A. R. Mitchell
Keyword(s):  
Bond Length ◽  
Surface Structure ◽  
Order Relation ◽  
Normal Incidence ◽  
Intensity Analysis ◽  
Ion Scattering ◽  
Low Energy Electron Diffraction ◽  
Adsorption Sites ◽  
Low Energy Electron ◽  
A Current

A new intensity analysis with low-energy electron diffraction is reported for the (2 × 2) surface structure obtained by the adsorption of H2S on the (111) surface of nickel. Intensity-versus-energy curves were measured with a video LEED analyzer for 10 diffracted beams at normal incidence, and comparisons were made with intensity curves calculated with multiple-scattering methods for models in which S atoms chemisorb at three-fold coordinated adsorption sites, but with the possibilities of both lateral and vertical relaxations in the local metallic structure. Small adsorbate-induced relaxations are found, but the dominant structural feature is that the S atoms adsorb above the "expected" adsorption sites (i.e. those which continue the regular fee packing) with a 1.50 Å spacing between the S layer and the top-most Ni layer. The S–Ni bond length of 2.10 Å agrees to within 0.02 Å of a prediction using a current bond length – bond order relation, but this value is smaller than two other recent measurements by SEXAFS and ion scattering by 0.06 and 0.10 Å, respectively. This analysis also finds the first two Ni layer spacings are expanded from the bulk value by 2 to 3%. Keywords: LEED, surface structure, S chemisorption, Ni(III) surface.

Further observations with low-energy electron diffraction for the Cu(100)-(2 × 2)-S surface structure: spot-profile and multiple-scattering analyses

1987 ◽  
Vol 65 (5) ◽  
pp. 500-504 ◽  
Author(s):  
H. C. Zeng ◽  
K. A. R. Mitchell
Keyword(s):  
Electron Diffraction ◽  
Surface Structure ◽  
Polar Angle ◽  
Low Energy ◽  
Energy Electron ◽  
Angle Of Incidence ◽  
Low Energy Electron Diffraction ◽  
Adsorption Sites ◽  
Low Energy Electron ◽  
Cu Interlayer

This study involves analyses by low-energy electron diffraction (LEED) for surface structures formed by S adsorbed on the (100) surface of copper. A LEED spot-profile investigation for a surface that shows a (2 × 2) diffraction pattern, supplemented by the effects of antiphase scattering, indicates that the domain boundaries do not correspond to microregions with local c(2 × 2) structure but rather that the beam elongations observed are consistent with local regions of the c(4 × 2) type in the approach to ¼ monolayer coverage. Diffracted-beam intensity-versus-energy curves calculated for the (2 × 2), c(2 × 2), and (2 × 1) translational symmetries, for fixed adsorption sites and S–Cu interlayer spacings, show that the intensity curves of corresponding beams can remain closely independent of actual symmetry and coverage even as the polar angle of incidence θ departs from the normal (although differences between the curves do tend to increase with θ). This observation can help simplify calculations of LEED intensities from adsorption systems with large unit meshes when the adsorbed species are in a constant environment; also, it provides an economical route for checking values of θ estimated from positions of diffraction spots on conventional LEED screens. When the latter is tested on the off-normal intensity data used in our previous analysis of the Cu(100)-(2 × 2)-S surface structure (Surf. Sci. 177, 329 (1986)), θ is indicated to be modified by 1° from the previously estimated value, but this does not significantly affect the determined S–Cu nearest neighbour bond length.

Inclusion of off-normal incidence data in a low-energy electron diffraction crystallographic analysis for the Cu(100)-(2 × 2)-S surface structure

1990 ◽  
Vol 68 (4-5) ◽  
pp. 353-356 ◽  
Author(s):  
H. C. Zeng ◽  
R. A. McFarlane ◽  
K. A. R. Mitchell
Keyword(s):  
Electron Diffraction ◽  
Surface Structure ◽  
Polar Angle ◽  
Normal Incidence ◽  
Crystallographic Analysis ◽  
Low Energy ◽  
Energy Electron ◽  
Low Energy Electron Diffraction ◽  
Low Energy Electron ◽  
Cu Interlayer

A low-energy electron diffraction (LEED) crystallographic analysis has been undertaken to assess the lateral and vertical relaxations for the Cu(100)-(2 × 2)-S surface structure. The study uses five normal-incidence beams and seven beams with an off-normal direction for a polar angle of incidence equal to 8°. Both lateral and vertical relaxations in the copper structure are small (0.03 Å or less) compared with the structure in bulk copper, but the senses are unchanged from the recent normal-incidence analysis (H. C. Zeng, R. A. McFarlane, and K. A. R. Mitchell, Phys. Rev. B, 39, 8000 (1989)). The new LEED-determined S—Cu bond length is 2.23 ± 0.06 Å, while the S to topmost Cu interlayer spacing is 1.28 ± 0.03 Å.

CHEMICAL RECONSTRUCTION OF THE TiAl(010) SURFACE

1995 ◽  
Vol 02 (02) ◽  
pp. 183-189 ◽  
Author(s):  
C.P. WANG ◽  
S.K. KIM ◽  
F. JONA ◽  
D.R. STRONGIN ◽  
B.-R. SHEU ◽  
...  
Keyword(s):  
Electron Diffraction ◽  
Atomic Structure ◽  
Atomic Layer ◽  
Interlayer Spacing ◽  
Low Energy ◽  
Intensity Analysis ◽  
Low Energy Electron Diffraction ◽  
Surface Phases ◽  
Low Energy Electron ◽  
Bulk Structure

The atomic structure of a clean (010) surface of the ordered binary alloy TiAl (with tetragonal bulk structure of the CuAu I type) is studied with quantitative low-energy electron diffraction (QLEED). Two different surface phases are found depending on the preparation procedure. After a cleaning step in vacuo by means of Ar-ion bombardments, anneals at 750−850°C produce a 2×1 surface and anneals at about 900° C produce a 1×1 surface. A QLEED intensity analysis of the 1×1 structure reveals the occurrence of chemical reconstruction, whereby the Ti atoms in the first layer exchange places with the Al atoms in the second layer. Thus, while any bulk (010) plane contains 50% Al and 50% Ti , the top atomic layer of a (010) surface contains 100% Al and the second atomic layer contains 100% Ti . Both layers are slightly buckled and the first interlayer distance is compressed about 7.1% while the second interlayer spacing is expanded about 7.4% with respect to the bulk value.

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