Periodic domain boundary ordering in a dense molecular adlayer: Sub-saturation carbon monoxide on Pd(111)

Abstract

We describe a previously unreported ordered phase of carbon monoxide adsorbed on the (111) facet of single crystal palladium at near-saturation coverage. The adlayer superstructure is identified from low energy electron diffraction to be c( ) 16×2 with respect to the underlying Pd(111) surface net. The ideal coverage is determined to be 0.6875 ML, approximately 92% of the 0.75-ML saturation coverage. Density functional theory calculations support a model for the molecular packing characterized by strips of locally-saturated ( ) 2×2 regions, with the CO bound near high-symmetry surface sites, separated by antiphase domain boundaries. The structure exists in a narrow coverage range and is prepared by heating the saturated adlayer to desorb a small fraction of the CO. Comparison of the c( ) 16×2 domain-boundary structure with structural motifs at lower coverages suggests that between 0.6 and 0.6875 ML the adlayer order may be more strongly influenced by interadsorbate repulsion than by adsorption-site-specific interactions. The system is an example of the structural complexity that results from the compromise between adsorbate-substrate and adsorbate-adsorbate interactions.