A low-temperature scanning tunneling microscope operated at 4.7 K was used to observe individual molecules of ethylene on the Pt(111) and Pd(110) surfaces following adsorption at 50 K. In both cases, two forms of the molecules were observed and were found to switch from one to the other under the influence of the tip. The form with the greater apparent height is attributed to π-bonded ethylene, and the other form is attributed to di-σ bonded ethylene. The images provide direct evidence that on both surfaces π-bonded ethylene occupies an on-top site and di-σ bonded ethylene occupies a symmetric bridging site. The results are compared with predictions based on DFT calculations that di-σ bonded ethylene should be the most stable form on both surfaces.
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