Electron diffraction at crystal surfaces: IV. Computation of LEED intensities for “muffin-tin” models with application to tungsten (001)
Jennings, P.J. and McRae, E.G. (1970) Electron diffraction at crystal surfaces: IV. Computation of LEED intensities for “muffin-tin” models with application to tungsten (001). Surface Science, 23 (2). pp. 363-388.
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Low energy electron diffraction intensities have been computed for “muffin-tin” models representing atomically-clean crystal surfaces and surfaces formed by the adsorption of foreign atoms. Computational results are reported for specific model potentials corresponding to tungsten crystal with a (001) surface and hydrogen adsorbed on tungsten (001) to form various surface structures of centered (2 × 2) periodicity. No attempt has been made to optimize the model potentials or to allow for inelastic scattering processes. The computational method is a combination of the generalized Darwin method (Part I in the series) with Kambe's method for simple monolayers. The scattering ability of tungsten and hydrogen atom layers and the appropriate band-structure section for tungsten are determined at intermediate stages in the intensity computation. Results are reported for the energy range 0–4 ryd (0–54 eV).
The main features of the computed intensity curve for tungsten appear to be related more directly to scattering resonances of a single layer of atoms than to the Bragg conditions referring to the free-electron description. The presence of a surface potential barrier has a substantial effect on intensity curves. The computational results are compared with the observed intensity versus energy curve for tungsten (001) in the energy range 0–20 eV. The computation accurately locates two peaks observed at 8 and 17 eV, but the model cannot account for a third peak observed at 4 eV. An interpretation as a displaced resonance of the surface monolayer is suggested.
The computations for hydrogen on tungsten indicate that integral-order beam intensities are only slightly affected by a hydrogen layer. The computed intensities for fractional-order beams are comparable with the intensities for neighboring integral-order beams, and are extremely structure-sensitive. The chief mechanism for the appearance of fractional-order beams is inter-layer multiple scattering.
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