Interactions between adsorbed molecules on metal surfaces

The tilted geometry of CO adsorbed on Pd(110) is identified using high-resolution electron energy-loss spectroscopy (HREELS), based on the observation of hindered rotational vibrations in the dipole-scattering spectra. 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 dispersion of hindered translational modes of CO molecules in the (2 x 1)p2mg structure on Pd(110) clearly reflects a short-range interaction between the CO molecules adsorbed at the nearest-neighbor sites. From the analysis of the distinctive dispersion, a strong repulsive force constant is obtained, i.e. 10.5 +/- 1.5 N/m, which quantitatively asserts that the tilted CO is formed by the steric repulsion between the nearest-neighbor molecules.