ANGULAR-DEPENDENCE OF THE EXFAS (EXTENDED FINE AUGER STRUCTURE) IN MGO(100) SURFACES - SHORT-RANGE ORDER VERSUS DIFFRACTION EFFECTS

We report the complete analysis of the angular dependence of the features observed in the MgO(100) surface measured at kinetic energies higher than the Auger L2,3VV line. The angular detection of the extended fine structure presents strong variations in both intensity and periodicity, showing a preferential emission along the low-index direction. On the contrary, in angular-integrated detection, the EXFAS features show remarkable similarities as a function of the azimuthal collection angle. In angular detection, the EXFAS signal consists of two overlapping electronic contributions. One has an EXAFS-like origin (as evidenced in polycrystalline samples) and the other, as pointed out in the present work, has a diffractive nature due to the forward-scattering effect experienced by the valence band electron escaping the crystalline surface.