Switching in the molecular orientation ruled by steric repulsion of adsorbed CO on Pd(110)

The molecular orientation of CO adsorbed on Pd(110) changes anisotropically depending on the ordered structures. The tilted geometry of CO is identified, using high-resolution electron energy-loss spectroscopy, based on the observation of hindered rotational vibrations in the dipole-scattering spectra in which the assignments are determined by the energy shifts of a series of CO isotopomers. With increasing coverage, the orientation of CO switches from the upright geometry to the tilted geometry first along the Pd row and then perpendicular to the row. The tilted geometry is controlled by how the nearest-neighbor CO-CO repulsion is reduced. [S0031-9007(99)08575-0].