Determination of the coverage of ethylene and ethylidyne on Pt(111) using elastic recoil detection analysis

Very high energy ions incident upon a sample with adsorbed light adatoms will cause some of the adsorbates to recoil off the surface with substantial amounts of energy. The number of recoil particles is a direct measure of the adatom concentration. Using this technique (elastic recoil detection analysis or ERDA), we have determined the concentration of adsorbed ethylene (C2H4) and ethylidyne (C2H3) on Pt(111). We find, both for adsorption below and at room temperature, that the coverage is approximately one half of a monolayer, in agreement with earlier work using X-ray photoelectron spectroscopy. We propose that the adsorbed ethylidyne molecules form a honeycomb lattice.