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Photoelectron Diffraction

In electron diffraction, such methods are hardly past their infancy. During the last decade or so, considerable effort has been focussed on the holographic paradigm [1, 2, 3, 4]. This is perhaps most easily illustrated in the case of photoelectron diffraction (Fig. 1). The direct path of a photoelectron from the emitter to a detector may be identified with a reference wave, while that which reaches the detector via subsequent scattering from a nearby atom may be regarded as an object wave. A computer algorithm may be devised that effectively retraces the path of the object wave back to its last scatterer. Hence a 3D image may be reconstructed of the atoms in the immediate vicinity of the emitter.

The ability of the holography to recover the phase of an object wave (through a knowledge of the reference wave) is well known from its optical applications. Its applicability even in situations of strong multiple scattering derives from the fact that the back-propagation of the object wave takes place through vacuum and therefore cannot retrace any scattering event prior to the last one. In practice, holographic images are normally reconstructed from diffraction patterns of several different electron energies. The most important consequence of this is the elimination of the troublesome twin images of conventional holography.

Click here to see animated holographic reconstruction.

Fig. 2 shows a recent example of the use of holography to determine the structure of a single-monolayer Mn-Ni surface alloy. This alloy was grown by depositing half a monolayer of Mn atoms on a Ni(001) surface. The question was whether the Mn atoms were deposited as a surface layer on top of an un-reconstructed Ni substrate, or whether they were incorporated in some way into the substrate. A holographic image reconstructed from a set of diffraction patterns from electrons of a set of different energies photo-emitted from the Mn atom clearly indicates the latter, with just a slight buckling of the top Mn-Ni layer. This formed an excellent starting point for conventional model-based structure refinements.


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Next: Macromolecular X-ray Crystallography Up: Reconstructing the Atomic Architecture Previous: Introduction