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 stru cture analysis is used to determine the adsorption geometry from compa rison of experiment and theory. The fcc threefold hollow site is prefe rred 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 t op two substrate layers is detected which corresponds to a local expan sion of the first-to-second layer distance underneath the adsorbate by 0.08 angstrom as compared to the bulk value. The three substrate atom s forming the hollow site are closer to one another than in the bulk s tructure by 0.11 angstrom. A tilt of the carbon-carbon bond by 6-degre es seems to occur and could be indicative of a wagging vibration of th is bond. The hydrogen atoms do not contribute to the diffraction that yields the ordered pattern as indicated by the absence of hydrogen rel ated features in the experimental I-V curves, suggesting relatively fr ee rotation of the methyl group about the carbon-carbon axis. Addition ally, 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) LEE D pattern, is possible. The p(2 x 2) is favored in the structural anal ysis, giving an ethylidyne coverage of 0.25 monolayers.