Structural determination of a molecular adsorbate by photoelectron diffraction: Ammonia on Ni{111}

The adsorption of ammonia (NH₃) on the Ni{111} surface has been investigated by photoelectron diffraction. To determine the adsorption site and the distance above the Ni{111} surface, photoelectron diffraction spectra of the N 1s core level were recorded in the scanned energy mode for normal and two off-normal emission directions and compared with extensive multiple-scattering calculations. The best agreement and the lowest R factor were found for an atop site and a Ni-N bond length of 1.97 {}. The anisotropic vibrations of the emitter have been taken into account in the theory and the mean-square displacement for the vibrations of the ammonia molecule parallel to the surface was determined to be 0.04 A².