Determination of the adsorption geometry of ethylene on Ni{110} using photoelectron diffraction

Using scanned-energy mode photoelectron diffraction we have determined the adsorption geometry for the c(2 x 4) phase of ethylene on Ni(110) at satura tion coverage of 0.5 ML, and at a lower coverage of 0.2 ML when no long-ran ge ordered overlayer is seen. The structural optimisation is based on compa rison of the experimental data with the results of multiple scattering calc ulations. For the c(2 x 4) phase the two molecules per primitive unit mesh occupy slightly different low-symmetry sites approximately midway between s hort-bridge and atop. There is a significant tilt of the C-C axis with resp ect to the surface plane of about 10 degrees. The C-C axes of the molecules are preferentially aligned along the close-packed Ni rows, but are offset in (100) directions away from the ridges by about 0.2 Angstrom. At low cove rage the corresponding modulation spectra are Very similar to those of the ordered phase; detailed optimisation of the fits to these low coverage spec tra confirms that the local adsorption site is also one of low symmetry bet ween atop and bridge. (C) 1999 Elsevier Science B.V. All rights reserved.