STRUCTURAL STUDY OF THE NI(100)-(2X2)-C SURFACE BY TIME-OF-FLIGHT SCATTERING AND RECOILING SPECTROMETRY

The structure of the Ni{100}-2 x 2)-C surface has been investigated by time-of-light scattering and recoiling spectrometry (TOF-SARS), low-e nergy electron diffraction (LEED), and classical scattering simulation s, The surface was prepared by chemisorption of either carbon monoxide , benzene, or aniline onto a Ni{100} surface at 600 degrees C. The sca ttering flux from 4 keV of Ne+ and the recoiling flux of carbon from 4 keV of Ar+ were monitored as a function of the crystal azimuthal angl e. An efficient simulation method, which is based on the binary collis ion approximation and shadowing and blocking effects, has been develop ed and applied to simulation of the TOF-SARS azimuthal scans. Quantita tive values for the geometrical parameters were obtained by using a re liability (R) factor to compare the experimental and simulated azimuth al scans. The results are consistent with a surface structure which co nsists of 80% of a clock reconstructed (2 x 2)p4g phase and 20% of an unreconstructed (2 x 2) phase. For the reconstructed phase, the Ni ato ms rotate in alternating clockwise and anticlockwise directions with t he Ni atoms displaced laterally from their original positions by 0.6 /- 0.1 Angstrom and the C atoms in the 4-fold sites at positions 0.1 /- 0.1 Angstrom above the plane of Ni atoms. The consequences of these results with respect to heterogeneous catalysis are discussed.