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Abstract

Scanned energy mode photoelectron diffraction spectra have been recorded from the C and N 1s levels of the adsorbate in the Ni(100)(2 × 2)-C(p4g) and Ni(100)(2 × 2)-N(p4g) structures at polar emission angles of 0°, 18° and 44° and are compared with the results of model calculations. The results allow a determination of the structure of these two phases which appear to be identical to within an estimated accuracy of a few hundreths of an ångström. The analysis supports the “clock reconstruction” model of these surfaces with the adsorbate atoms located in 4-fold coordinated hollows at the centres of rotation of the top layer Ni atom movements parallel to the surface. The magnitude of these parallel displacements is found to be 0.55 ± 0.20 Å, and is accompanied by a top layer expansion of 0.15 ± 0.10 Å. The adsorbate atoms lie 0.25 ± 0.05 Å above the location of the top layer in the absence of expansion, and are thus 0.10 ± 0.12 Å above the expanded top layer. Within these parameter ranges there is some evidence that the degree of distortion (particularly perpendicular to the surface) may be slightly larger in the N phase than in the C phase. These values compare very well with LEED analyses, although the result for the N appears to be at variance with SEXAFS results.