Lateral interactions of CO in the (2×1)p2mg structure on Pd(110): Force constants between tilted CO molecules

Intermolecular interactions between tilted CO molecules in the $\text{(2×1)p2mg}$ structure on Pd(110) were studied by means of high-resolution electron energy-loss spectroscopy (HREELS). We measured the distinctive dispersion of the C–O internal stretching mode and the hindered translational modes. The dispersion of the C–O internal stretching mode is well explained by the dynamic dipole–dipole interaction. On the other hand, the dispersion of hindered translational modes is anisotropic. In contrast to the constant $T_{\text{[001]}}$ mode along [001], both the $T_{\text{[001]}}$ and $T_{\text{[11̄0]}}$ modes observed along $\text{[11̄0]}$ are dispersive. The dispersive feature of the hindered translational mode along $\text{[11̄0]}$ was not observed for twofold periodic CO in a $\text{p(2×1)}$ structure close to half coverage on Pd(110). Therefore, the anisotropic dispersion 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., $\text{10.5±1.5\hspace{0.05em}}\mathrm{N/m},$ which quantitatively asserts that the tilted CO is formed by the steric repulsion between the nearest neighbor molecules.