A LEED-IV DETERMINATION OF THE RU(001)-P(2X2)(O- A COADSORBATE-INDUCED MOLECULAR TILT(CO) STRUCTURE )

A detailed LEED-IV analysis of the coadsorbate system p(2 X 2)(O + CO) /Ru(001) has been performed by conventional dynamical LEED calculation s and by Tenser LEED. We find that in this coadsorbate layer the oxyge n atoms remain in the threefold hcp sites as they occupy in the pure p (2 x 2)O structure with minimally changed distances, in contrast to th e behaviour of the p(2 X 2)(2O + CO) structure where a structural rear rangement of the oxygen atoms has been observed before. The CO molecul es adsorb in the top site in the center of three O atoms. Their molecu lar bond length is 1.16 + +/- 0.06 Angstrom i.e., insignificantly long er than in the free molecule; the bond length to the underlying Ru ato m is d(C-Ru) = 1.93 +/- 0.06 Angstrom. Buckling of the top Ru layer si milar to that for the p(2 x 2)(2O + CO) system is found. A dramatic re duction of the best-fit R-factors (for the Pendry R-factor of 0.33-0.1 7, with particularly strong effect on the integral-order beams) can be achieved by the introduction of a tilt angle for the CO molecules, wi thout any other structural changes. The resulting tilt lies in the thr ee equivalent symmetry planes through each of the O atoms; the molecul e is tilted away from the O atom in this plane. At 120 K, the best-fit tilt angle is 12.6 degrees. Different domain averaging procedures do not change these conclusions. The physical meaning of these findings a nd the possible reasons for the tilt and for the high sensitivity of t he structure determination to it are discussed.