C2H4 ADSORPTION ON THE NI(111) FACE AT ROOM-TEMPERATURE - VIBRATIONS OF SURFACE COMPLEXES STUDIED BY ELECTRON-ENERGY LOSS SPECTROSCOPY

Ethylene adsorption at room temperature on the Ni(111) surface has been followed by vibrational high resolution electron energy loss spectroscopy coupled with LEED, work function changes and thermodesorption measurements. The vibrational spectra of adsorbed species exhibit loss peaks at 365, 151, 85, 60 and 39 meV (274, 147, 79 and 39 meV with C2D4). The similarity of these spectra with those obtained after short exposure to acetylene suggests a dehydrogenating adsorption well confirmed by thermodesorption data: C2H4 gas→ C2H2ads + 2 Hads. From the observed vC-C frequencies (151 and 147 meV for the respective C2H2 and C2D2 adsorbed species) one can deduce a residual force constant f{hook}C-C ≅ 5.25 mdyn/Å. By comparison to gaseous molecules, this value suggests a near sp3 hybridization state of the C atoms confirmed by the position of the vC-H stretching mode (365 meV). Taking in account these results, a triangular surface chemisorption site for the C2H2 species with two σ bonds and one π bond to three Ni atoms is suggested. Frequency analysis of this model allows to fit the experimental loss energies for reasonable force constant values, the C2H2 residue lying flat on the triangular site with a bent H-C-C-H arrangement (C-C-H angle ≅ 155°). © 1979.