Uracil on Cu(110) : a quantitative structure determination by energy-scanned photoelectron diffraction

The local adsorption site of the nucleobase uracil on Cu(110) has been determined quantitatively by energy-scanned photoelectron diffraction (PhD). Qualitative inspection of the O 1s and N 1s soft x-ray photoelectron spectra, PhD modulation spectra, and O K-edge near-edge x-ray adsorption fine structure indicate that uracil bonds to the surface through its nitrogen and oxygen constituent atoms, each in near atop sites, with the molecular plane essentially perpendicular to surface and aligned along the close packed [1 (1) over bar0] azimuth. Multiple scattering simulations of the PhD spectra confirm and refine this geometry. The Cu-N bondlength is 1.96 +/- 0.04 angstrom, while the Cu-O bondlengths of the two inequivalent O atoms are 1.93 +/- 0.04 angstrom and 1.96 +/- 0.04 angstrom, respectively. The molecule is twisted out of the [1 (1) over bar0] direction by 11 +/- 5 degrees