Photoelectron-diffraction analysis of the structure ofc(2 × 2)O on Ni(001)

Calculated energy-dependent photoelectron-diffraction (EDPD) curves for oxygen-nickel perpendicular spacings (d_⊥) between 0.0 and 1.2 {} are presented and compared with experimental data for the c (2 ×{} 2)O-Ni(001) system. Using a normalized R factor, we found that the only acceptable agreement is for d_⊥ spacings in the range 0.78-0.90 {}. The experimental observation that adsorbate-induced modulations remain strong over a wide energy range in EDPD curves, but not in low-energy electron-diffraction intensity curves, is explained.