Determination of chemisorption geometries for complex molecules by using near-edge x-ray absorption fine structure: Ethylene on Ni(100)

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Abstract
We have derived theoretical expressions for the incident beam angular dependence of the absorption cross section for ethylene chemisorbed on single crystal surfaces with linearly polarized light. The changes in the π and σ resonance intensities for C 1s near-edge x-ray absorption fine structure (NEXAFS) spectra can be related to the geometry of chemisorption relative to the surface. The derived equations have been used to analyze experimental data in order to determine the structure of chemisorbed ethylene on Ni(100). At low temperatures ethylene bonds with the molecular axis parallel to the surface but with the molecular plane tilted 50°. The geometry of vinyl fragments formed by annealing the crystal to 180 K was also determined. The C–C bond is bent 35° from the surface normal in this case, with the plane of the molecule tilted by 65°.