Photoelectron holography studies of Bi on Si(111)
Photoelectron holography is applied to Bi atoms adsorbed on Si(111) for a determination of the three-dimensional atomic structure. Instead of measuring the absolute intensity as a function of photon energy as is traditionally done, a differential measurement is carried out to yield the logarithmic derivative of the intensity. This leads to intensity self-normalization, and significantly improves the accuracy. The resulting images show a well-resolved trimer structure for the adsorption geometry.
Imaging of a surface alloy with energy-dependent photoelectron holography.
Fourier transformation of experimental energy-dependent photoelectron diffraction data has been used to produce an essentially artifact-free image of a surface alloy. This direct method, based upon the intersection off contour arcs associated with each measurement direction, can provide vectorial atomic positions with atomic resolution. A rapid data collection mode was introduced. Surface structural sensitivity was confirmed by comparison with multiple-scattering simulations. The previously ambiguous surface geometry of [ital c](2[times]2) Au/Cu(001) has been determined, with clear, non-model-dependent discrimination of the surface alloy over the overlayer structure.
Mg on Pd(111): The formation of local order observed by photoelectron diffraction.
We have studied the growth of Mg overlayers on Pd(111), a system with 16.6% misfit, by means of x-ray photoelectron diffraction. No long-range order is observed by low-energy electron diffraction. Pronounced forward-focusing maxima in Mg 1s emission indicate considerable short-range order. At coverages below 1 monolayer Mg is found to occupy fcc sites distributed over two layers. At higher coverages oriented clusters are formed, accommodating the misfit by alternatively occupying fcc and hcp sites. Comparison to single-scattering calculations suggests an average cluster size of less than 15 A