Extended Appearance-Potential Fine-Structure Analysis: Oxygen on Al(100)

To measure O-Al separation at Al(100) surfaces disordered (low-energy electron-diffraction beams extinguished) by reaction with oxygen, the extended appearance-potential fine structure was analyzed above the threshold for electron-bombardment excitation of the O $1s$ core. Calculation shows that the outgoing electron has angular momentum $l=0\mathrm{}$, allowing simple Fourier inversion of the fine structure. The separation, 1.98±0.05 Å, suggests that oxygen lies under the top layer, a result undetectable in extended-x-ray-absorption fine-structure measurements on thicker films.