The coverage dependence of the local structure of C on Ni(100): a structural precursor to adsorbate-induced reconstruction

The local structure around adsorbed carbon atoms on Ni(100) has been determ ined by C Is scanned-energy mode photoelectron diffraction both at low cove rage (0.15 ML) and at 0.5 ML in the 'clock'-reconstructed (2 x 2)p4g phase. At low coverage the C atoms occupy simple undistorted hollow sites; the ra dial strain of the Ni atoms surrounding the adsorbed carbon, proposed on th e basis of STM images, is not found. The comparative study of the (2 x 2)p4 g phase confirms the structural parameters of earlier studies, but allows d irect comparison between the two phases free from systematic errors. The C- Ni near-neighbour distances to both top and second layer Ni atoms are uncha nged by reconstruction, but the Ni-Ni nearest-neighbour distance in the top layer increases significantly. This implies that the adsorbate-induced com pressive stress is associated with Ni-Ni, rather than Ni-C, repulsion as ha s previously been proposed. Special care has been taken to check for coupli ng of structural parameters in the analysis which might influence the preci sion of these comparisons, and a new methodology for identifying and assess ing such parameter coupling is described. (C) 2000 Elsevier Science B.V. Al l rights reserved.