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Surface X-Ray Diffraction

A similar idea allows the direct recovery of surface electron density from measured intensities of reciprocal lattice rods in surface X-ray diffraction. Fig. 5 is a schematic of a diffraction pattern from a (2x2) reconstructed InSb(111) surface. The reciprocal lattice rods run through the Bragg spots on this diagram, and perpendicular to the plane of the figure. It will be noted that the diffraction amplitudes of the integer-order rods are a sum of those from the bulk and surface regions of the sample, while the fractional-order rods arise solely from diffraction from the reconstructed surface region.

The integer-order rods may be treated in a manner entirely analogous to the structure completion problem described in the previous section. The bulk is the known part of the structure and the surface the unknown part. However, inclusion of just the integer-order (crystal truncation) rods alone in the reconstruction algorithm gives only the average structure (Fig. 6a) within the surface unit cell (i.e. one which has the bulk 2D periodicity). Nevertheless, this gives an excellent starting electron density for further refinement by inclusion in the algorithm of the fractional-order rod intensities, which finally recovers the accepted vacancy-buckling model [12] in which the Sb atoms in the top double layer closest to the vacancies have moved towards those holes. Such a tendency is apparent even after just 10 iterations of the algorithm following the inclusion of the fractional-order intensities (Fig. 6b). The final reconstructed image (not shown) is even capable of resolving bucklings normal to the surface of as little as 0.2 Å.


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