THE O NI(111)-P(2 X-2) STRUCTURE - ADSORBATE-INDUCED RELAXATIONS/

The adsorption geometry and the adsorbate induced relaxations of p(2 x 2) ordered oxygen on Ni(111) have been reinvestigated by a dynamical LEED analysis in the energy range between 30 and 387 eV. 14 non-equiva lent beams have been recorded with a total energy range of 2830 eV. Ve rtical distances, buckling and lateral shifts including rotations have been tested for the adsorbate and the first 2 substrate layers. Top a nd the three-fold coordinated hcp and fcc sites were tested as adsorpt ion sites. While the top site can be excluded already by tests on the non-buckled surface the discrimination between hcp and the strongly fa voured fcc sites is only possible if buckling is included in the model calculations. Optimal agreement between calculated and experimental c urves (R(p) = 0.226) was obtained for a Ni-O nearest neighbour distanc e of 1.83 angstrom. Significant buckling up to 0.09 angstrom was found in the first 2 substrate layers, but the average Ni-Ni layer distance is only slightly (0.02 angstrom) expanded between the first and secon d substrate layer, and corresponds to the bulk value (2.03 angstrom) f or the other layers. Adsorbate induced lateral shifts (0.02 +/- 0.04 a ngstrom for nearest neighbours of O) turned out to be at the limit of detectability already in the first substrate layer. No indication coul d be found for adsorbate induced rotational shifts in the topmost subs trate layers.