THE ADSORPTION OF SMALL HYDROCARBONS ON CU(111) - A COMBINED HE-ATOM SCATTERING AND X-RAY-ABSORPTION STUDY FOR ETHANE, ETHYLENE, AND ACETYLENE

Ethane (C2H6), ethylene (C2H4), and acetylene (C2H2) adsorbed on Cu (1 11) are investigated using high-resolution helium atom scattering and x-ray absorption spectroscopy (NEXAFS). For C2H6/Cu(111) and C2H4/Cu(1 11) the excitation energies of the frustrated molecular translation no rmal to the surface (FTz) amount to 6.7 meV, suggesting the presence;o f a physisorbed species which is consistent with the NEXAFS data for e thylene, In contrast, for C2H2/Cu(111) the NEXAFS data indicate strong intramolecular distortions of the acetylene adsorbate compatible with a tilt of both CH ends away from the molecular axis. While the latter finding is in agreement with recent theoretical studies the theoretic ally predicted chemisorbed ethylene species could not be observed by t he experiment. However, more detailed theoretical studies of the ethyl ene-Cu(lll) interaction potential reveal two minima separated by an ac tivation barrier. The minimum closer to the surface refers to strongly distorted chemisorbed C2H4 whereas the outer minimum is characterized by a free molecule-like physisorbed species. Thus the results from th e present measurements are explained by the theoretically confirmed ph ysisorbed species while chemisorbed C2H4 has to be excluded. Complemen tary results for ethylene and acetylene adsorbed on Pb(lll) reveal a F Tz-mode energy of 6.5 and 6.7 meV, respectively, thus revealing a much weaker acetylene-substrate binding than seen for Cu(111). Also in cas e of Pb(111) the FTz-mode showed an Einstein-like behavior with a flat dispersion curve, as for corresponding modes on the Cu(111)-substrate , see above. (C) 1998 American Institute of Physics.