Geometric structure and chemical bonding of acetylene adsorbed on Cu(1 1 0)

The chemical bonding and geometric structure of acetylene adsorbed on Cu(1 1 0) is analyzed using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) in combination with density functional theory (DFT) total energy geometry optimizations and spectral calculations. XPS reveals two different adsorbed species at liquid nitrogen temperature. The molecular alignment is deduced from angle-resolved XAS, revealing that in one site the molecules are aligned with the C–C axis along the [0 0 1] direction and in the other site with an average angle of 35° to the Cu rows. The position of the shape resonance is used to deduce a C–C bond length of 1.35 Å, which is close to the values obtained from the DFT geometry optimizations. XES reveals strong σ–π mixing and new occupied states close to the Fermi level, originating from the out-of-plane π* orbital, which becomes occupied upon adsorption in agreement with the Dewar–Chatt–Duncanson model of the bonding.