Ordered structures of CO on Pd(1 1 1) studied by STM

The (3×3)R30°, c(4×2)-2CO, (2×2)-3CO, and several intermediate coverage structures of CO on Pd(1 1 1) have been studied by scanning tunneling microscopy. Shifts of the CO binding site with increasing coverage are observed. At coverages of 1/3 ML and below, CO occupies threefold hollow sites. Near 1/2 ML, regions of c(4×2)-CO with both fcc and hcp threefold hollow site occupation coexist with bridge bonded c(4×2)-CO. Above 1/2 ML, a progression of ordered phases with complex unit cells is observed. Near 3/4 ML, bright maxima appear with (2×2) periodicity. Simulated images based on the density functional theory optimized geometry indicate that these highly corrugated maxima are due to CO molecules bound to top sites, which obscure the other two CO molecules per unit cell. In addition to the symmetric 2×2 structure, previously unreported domains of lower symmetry 2×2 are observed.