Direct structural information from X-ray photoelectron diffraction: intermixing and on-surface adsorption of Na on Al surfaces

We have investigated the local structures of the room-temperature Na adsorption phases on Al(001) and (111) surfaces with X-ray photoelectron diffraction. Angular distributions of X-ray excited Na 1s photoelectrons and Na KVV Auger electrons emitted from c(2 × 2)-Na/Al(001), (√3 × √3)R30°-Na/Al(111) and p(2 × 2)-Na/Al(111) have been measured over the full solid angle above the Al surfaces. We show that angle-scanned photoelectron diffraction, in contrast to other structural techniques, gives very direct information on the degree of intermixing at the surface. Many structural models can be ruled out directly from the experimental results, and for the case of intermixing the interface structure can be determined by simple geometrical considerations.