A PHOTOELECTRON DIFFRACTION STUDY OF THE NI(100)(2 X 2)-C(P4G) AND NI(100)(2 X 2)-N(P4G) STRUCTURES

Scanned energy mode photoelectron diffraction spectra have been recorded from the C and N 1s levels of the adsorbate in the Ni(100)(2 x 2)-C(p4g) and Ni(100)(2 x 2)-N(p4g) structures at polar emission angles of 0-degrees, 18-degrees and 44-degrees 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 angstrom. 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 angstrom, and is accompanied by a top layer expansion of 0.15 +/- 0.10 angstrom. The adsorbate atoms lie 0.25 +/- 0.05 angstrom above the location of the top layer in the absence of expansion, and are thus 0.10 +/- 0.12 angstrom 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.