A LEED-IV determination of the Ru(001)-p(2 × 2) (O + CO) structure: A coadsorbate-induced molecular tilt

A detailed LEED-IV analysis of the coadsorbate system p(2× 2)(O + CO) Ru(001) has been performed by conventional dynamical LEED calculations and by Tensor LEED. We find that in this coadsorbate layer the oxygen atoms remain in the threefold hcp sites as they occupy in the pure p(2 × 2)O structure with minimally changed distances, in contrast to the behaviour of the p(2 × 2)(2O + CO) structure where a structural rearrangement of the oxygen atoms has been observed before. The CO molecules adsorb in the top site in the center of three O atoms. Their molecular bond length is 1.16 ± 0.06 A ̊, i.e., insignificantly longer than in the free molecule; the bond length to the underlying Ru atom is d(C-Ru) = 1.93 ± 0.06 A ̊. Buckling of the top Ru layer similar to that for the p(2 × 2)(2O + CO) system is found. A dramatic reduction of the best-fit R-factors (for the Pendry R-factor of 0.33-0.17, with particularly strong effect on the integral-order beams) can be achieved by the introduction of a tilt angle for the CO molecules, without any other structural changes. The resulting tilt lies in the three equivalent symmetry planes through each of the O atoms; the molecule is tilted away from the O atom in this plane. At 120 K, the best-fit tilt angle is 12.6°. Different domain averaging procedures do not change these conclusions. The physical meaning of these findings and the possible reasons for the tilt and for the high sensitivity of the structure determination to it are discussed.