Molecular Adsorbate Structures from Angular-Resolved Photoemission: Ammonia on Ir(111)

Strong azimuthal effects have been observed in the angular-resolved photoemission from the $1e$ orbital of N${\mathrm{H}}_3$ chemisorbed on Ir(111). Some of the anisotropy is clearly due to substrate scattering. Other spectral features appear to be dominated by initial-state effects from the N-H bonds. Analysis of the diffraction features based on near-neighbor substrate scattering shows the bonding site to be the $\mathrm{ABC}$ site, i.e., a threefold hollow with a vacancy underneath.