Photoelectron Holography Applied to Surface Structural Determination.

Photoemitted electron waves are employed as coherent source waves for angstrom-scale holographic imaging of local atomic geometry at surfaces. Electron angular distribution patterns with a specific electron kinetic energy are collected above a sample surface and serve as a record of the interference between the source wave and the waves scattered from surrounding ion cores. Upon application a mathematical imaging integral transformation, the three-dimensional structural information is obtained directly from these collected patterns. Patterns measured with different electron kinetic energies are phase -summed for image improvement. A platinum (111) clean metal surface is used as a model system to experimentally verify the technique. A pattern measured at k = 9.6A^{-1} (351 eV electron kinetic energy) is used to generate a full three-dimensional image of atom locations around an emitter with nearest neighbors within 0.1A of the expected bulk positions. Atoms several layers beyond the nearest neighbors are also apparent. Twin-image reduction and artifact suppression is obtained by phase-summing eight patterns measured from k = 8.8 to 10.2A^ {-1} (295 to 396 eV). In total, thirty -two patterns were measured in 0.2A^{ -1} steps from k = 6.0 to 12.2A^ {-1} (137 to 567 eV) and are presented here. Simple models of two-slit interference are compared with the electron scattering process to illuminate our understanding of the holographic recording of the structural information. This view also shows why the technique occasionally fails due to destructive interferences. Simple theoretical models of electron scattering are compared to the experimental data to show the origin of the structural information and the differences that result from the atomic scattering process and from the nature of the source wave. Experimental parameters and their relationship to imaging is discussed. A detailed comparison is made to the platinum pattern measured at 351 eV using the simple theoretical model. The remaining data set is also modeled, and the eight appropriate theoretical patterns are used to regenerate the multiple-wave number experimental result. A second metal system of a clean Cu (001) surface is also measured and imaged using the same technique. Using the copper system as an example, a study is done to show how this type of holography can have a tendency to favor the imaging of forward scattering atoms. This confirms the ability of the technique to be used in buried overlayer or interface studies, in addition to overlayer and bulk systems.