SURFACE CARBOXYLATE SPECIES ON CU(100) STUDIED BY ANGLE-SCANNED PHOTOELECTRON DIFFRACTION AND LCAO-LDF CALCULATIONS

Chemisorption site geometries of formate and acetate species on Cu(100) have been studied by means of angle-scanned X-ray photoelectron diffraction (XPD) and first-principle quantum-mechanical calculations on a Cu60-formate cluster in the framework of local density functional theory (LDF). According to our LDE calculations, the short bridge site results to be more stable than the cross bridge one by about 20 kcal/mol. The reasons for the more effective interaction of the adsorbate in the short bridge site have been clearly outlined on the basis of the analysis of the LDF wavefunctions. LDF-optimized structural parameters have been used as a starting input in a series of single-scattering cluster spherical wave (SSC-SW) simulations of the O 1s XPD curves, which have experimentally confirined the occupation of the short bridge site proposed by LDF calculations. The SSC-SW simulations have also outlined the role of the low-frequency-hindered rotational modes of the adsorbate (librations) and have furnished an estimate of the frequency (approximately 70 cm-1) of the hindered rotation on the axis perpendicular to the surface. Furthermore, as regards the formate species, the experimental data give evidence of a Cu-O distance of 1.95 angstrom, in agreement with the LDF calculations and with angle-resolved photoemission fine structure (ARPEFS) literature results. An estimate of the OCO angle (129-degrees +/- 5-degrees) has been obtained from polar scans for both formate and acetate species.