ETHYLIDYNE ON PT(111) - DETERMINATION OF ADSORPTION SITE, SUBSTRATE RELAXATION AND COVERAGE BY AUTOMATED TENSOR LEED

The adsorption of ethylidyne on Pt(111) is investigated by low-energy electron diffraction (LEED). An automated search program for LEED structure analysis is used to determine the adsorption geometry from comparison of experiment and theory. The fcc threefold hollow site is preferred by the fit. The carbon-carbon bond distance is 1.49 angstrom and the carbon-platinum bond length 1.91 angstrom. A buckling within the top two substrate layers is detected which corresponds to a local expansion of the first-to-second layer distance underneath the adsorbate by 0.08 angstrom as compared to the bulk value. The three substrate atoms forming the hollow site are closer to one another than in the bulk structure by 0.11 angstrom. A tilt of the carbon-carbon bond by 6-degrees seems to occur and could be indicative of a wagging vibration of this bond. The hydrogen atoms do not contribute to the diffraction that yields the ordered pattern as indicated by the absence of hydrogen related features in the experimental I-V curves, suggesting relatively free rotation of the methyl group about the carbon-carbon axis. Additionally, a distinction between a p(2 x 2) and an average of three domains of a p(2 x 1) structure, which both give rise to the same (2 x 2) LEED pattern, is possible. The p(2 x 2) is favored in the structural analysis, giving an ethylidyne coverage of 0.25 monolayers.