Modeling of the geometry of densely packed chemisorbed overlayers: small organic molecules on Ni(110) and Pd(110)

The use of standardized force fields is suggested for modeling the geometries of chemisorption systems. In the case of densely packed overlayers with strong lateral interactions between the adsorbate molecules pertinent information on the structure may be obtained by considering only van der Waals interadsorbate contributions, augmented by terms from dipole and quadrupole interactions where applicable. Due to their localized nature, the van der Waals interactions may easily be evaluated using cluster models. This approach is exemplified for overlayers of CO, C₂N₂, C₆H₆ and C₂H₄ on the (110) surface of nickel and pallasium. The well-known geometry of Ni(110)/CO may be rationalized as a balance between intra- and inter-row repulsion of [110] rows of CO molecules. However, the bending angle NiCO is also affected by adsorbate-substrate interaction. For C₂N₂ on Pd(110) a planar c(2 × 2) arrangement with the molecular axis along the [001] direction is found, but for Ni(110) the strain caused by close packing is so large that tilting with respect to the (110) plane seems unavoidable. These findings are consistent with recent ARUPS data. The results for the adsorbates C₆H₆ and C₂H₄ are only indicative since the differences discriminating various geometries are too small to be conclusive in a model without adsorbate-substrate interactions. These findings show that the presented model approach provides a useful tool to investigate overlayer geometries and goes beyond the commonly used van der Waals hard-sphere models by qualitatively estimating the lateral interadsorbate interaction.