A LEED determination of the structures of Ru(001) and of CO/Ru(001)(3 3)R30
b'The structures of Ru(001) and of the \\u221a3 \\xd7 \\u221a3 R30\\xb0 overlayer of CO on Ru(001) have been determined by LEED I\\u2013V measurements and comparison to calculations. Special attention was paid to accurate angular alignment, selection of a well-ordered portion of the surface, and avoidance of beam-induced changes of the CO layer. Five orders of reflexes over a range of 300 eV each were used for the clean surface and 7 orders over 200 eV each for the CO superstructure. For the clean surface, a slight contraction of the first layer spacing (by 2%) was found which gave r-factors of 0.04 (Zanazzi-Jona) and 0.16 (Pendry) for 5 non-degenerate beams. For the CO structure the most probable geometry is the on-top site with spacings d(Ru---C) = 2.0 \\xb1 0.1 \\xc5andd(C---O) = 1.10 \\xb1 0.1 \\xc5 (rZJ = 0.21; rP = 0.51). The two threefold hollow and the bridge sites can be clearly excluded.'