THE INFLUENCE OF ELECTRONEGATIVE COADSORBATES ON THE GEOMETRY OF BENZENE ON RU(001)

The geometrical structures of the Ru(001)-p(3 x 3)-C6D6+2O and Ru(001) -p(3 x 3)-C6D6+2NO coadsorbate layers have been determined by a detail ed LEED IV analysis. The benzene molecule as well as the coadsorbates are found to be bound on hcp sites. The most conspicuous result is tha t the molecular symmetry is different for the two coadsorption layers. In the O coadsorbate layer it is essentially C-3v(sigma(v)), as for t he pure benzene layer, while NO coadsorption induces a rotation by sim ilar to 20 degrees towards C-3v(sigma(d)). In addition, the benzene mo lecule in the NO mixed layer has a distinct triangular distortion towa rds Kekule benzene. These changes are explained by spatial restriction s induced by the repulsive interaction between the benzene and the NO molecules, and by the electronic interactions between the benzene and the electronegative coadsorbate. Both coadsorbates occupy two inequiva lent hcp sires, which are discriminated by different local environment s with respect to the benzene molecule. We suggest that this leads to anisotropic indirect (through-substrate) interactions between the benz ene molecule and the respective coadsorbate. In addition, there may al so be direct attractive interactions between the coadsorbates. (C) 199 7 Elsevier Science B.V.