A new LEED intensity analysis of the reconstructed Au(110)-(1×2) surface results in a modification of the missing row model with considerable distortions which are at least three layers deep. The top layer spacing is contracted by about 20%, the second layer exhibits a lateral pairing displacement of 0.07 .~ and the third layer is buckled by 0.24 ,~. Distortions in deeper layers seem to be probable but have not been considered in this analysis. The inter-atomic distances in the distorted surface region show both an expansion and a contraction compared to the bulk value and range from 5% contraction to about 4% expansion.

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The reconstruction of the Si͑001͒-2ϫ1 surface consists of asymmetric and buckled Si dimers. The vertical separation between the up and the down atom within the dimer is about 0.72Ϯ0.05 Å and the dimer bond length of 2.24Ϯ0.08 Å has been found to be slightly smaller than the Si-Si distance in the bulk. The tilt of the dimer is 19Ϯ2°. The formation of Si dimers induces pronounced distortions in the substrate that were detectable down to the fifth Si layer. The structure determination is based on two independent low-energy electrondiffraction data sets taken in two different laboratories. The structural results agree well within the error limits, though noticeable differences occur between the experimental data sets. These differences in the experimental data can possibly be attributed to different preparation procedures.

The LEED pattern of the Au( 110) surface shows a (1 X 2) and also a (1 X 3) superstructure.The (1 X 2) superstructure has been determined by comparison of LEED intensities with model calculations. The missing row model is the most probable model. A miniium of the averaged F-factor, F = 0.36, has been found for 15% contraction of the first layer spacing without atomic displacements in the second layer.The clean (110) faces of gold are known to exhibit a reconstruction of the atomic arrangement in the surface layers with respect to the bulk structure. Until now a (1 X 2) superstructure with a doubling of the lattice spacing in [OOI] direction has been reported [1,2] ; now also a (1 X 3) superstructure has been found. Characteristic for both structures are diffuse LEED beams, significantly broadened in [OOl] direction. The angular half width of the beams depends on energy and angle of incidence. This behaviour has been interpreted previously for the (1 X 2) superstructure, applying a disordered missing row model with a slightly roughened surface [2,3]. This model can be used as well to explain the existence of the (1 X 3) superstructure.Similar superstructures have been observed at the (110) faces of platinum and iridium [4] and a first structure analysis by LEED for Ir (1 lO)(l X 2) [S] shows, that the missing row model is the most probable one. Here we present a LEED structure determination, assuming a well-ordered superstructure, neglecting the effect of the disorder of the (110) surface.The Au crystal of 5N purity has been oriented within 0.5" of the [ 1 lo] direction by X-ray diffraction and was cut and planed by spark erosion. Electrochemical etching in SN HzS04 followed by electrolytical polishing in a cyanide solution produced a smooth reflecting surface. After argon ion bombardment the LEED pattern exhibits a very diffuse (1 X 2) superstructure -fig. la -which coalesces after tempering up to 800 K into the pattern of a (1 X 2) or (1 X 3) structure.The conditions at which the (1 X 3) superstructure appears are not quite clear; it seems to be dependent on the special ion bombardment conditions and probably on the cooling rate after tempering. In both cases the spots are broadened in [OOl] *

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