The adsorption of small hydrocarbons on Cu(111): A combined He-atom scattering and x-ray absorption study for ethane, ethylene, and acetylene

Ethane ${\mathrm{(C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{6}}\mathrm{)},$ ethylene $({\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{4}}\mathrm{),}$ and acetylene $({\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{2}})$ adsorbed on Cu (111) are investigated using high-resolution helium atom scattering and x-ray absorption spectroscopy (NEXAFS). For ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{6}}\mathrm{/Cu(111)}$ and ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{4}}\mathrm{/Cu(111)}$ the excitation energies of the frustrated molecular translation normal to the surface $({\mathrm{FT}}_z)$ amount to 6.7 meV, suggesting the presence of a physisorbed species which is consistent with the NEXAFS data for ethylene. In contrast, for ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{2}}\mathrm{/Cu(111)}$ 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 theoretically predicted chemisorbed ethylene species could not be observed by the experiment. However, more detailed theoretical studies of the ethylene-Cu(111) interaction potential reveal two minima separated by an activation barrier. The minimum closer to the surface refers to strongly distorted chemisorbed ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{4}}$ whereas the outer minimum is characterized by a free molecule-like physisorbed species. Thus the results from the present measurements are explained by the theoretically confirmed physisorbed species while chemisorbed ${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{4}}$ has to be excluded. Complementary results for ethylene and acetylene adsorbed on Pb(111) reveal a ${\mathrm{FT}}_z$ -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 case of Pb(111) the ${\mathrm{FT}}_z$ -mode showed an Einstein-like behavior with a flat dispersion curve, as for corresponding modes on the Cu(111)-substrate, see above.