THE LOCAL ADSORPTION STRUCTURE OF ACETYLENE ON CU(111)

A quantitative structural study of the adsorption of acetylene on Cu(111) has been carried out by scanned energy mode photoelectron diffraction from the C ls core levels. The molecule is found to be adsorbed with the C-C axis almost parallel to the surface with the carbon atoms occupying the two symmetrically distinct three-fold coordinated hollow sites on the surface. The C to top Cu atom layer spacing appears to be slightly smaller (1.38 +/- 0.03 angstrom) above the ''fcc'' hollow site (above a third-layer Cu atom) than above the ''hcp'' hollow site (1.44 +/- 0.03 angstrom). The best fit to theory is obtained with the C atoms exactly in the three-fold symmetric sites of the substrate which constrains the C-C bondlength to a value (1.48 +/- 0.10 angstrom) substantially larger than that of the free molecule. This result is consistent with previously published vibrational spectroscopy from this adsorption system which indicates significant lowering of the C-C bond order.