CO on Ru(001): Island size and disordering

Low-energy electron diffraction (LEED) has been used to study the influence of steps and of temperature variation on the formation of ordered islands of CO on the (001) surface of ruthenium. The clean surface was found to have steps two atoms (one hcp unit cell) high separated by terraces with an average width between 135 and 270 Å. Widths of the beam profiles for the CO overlayer were measured as a function of coverage at 100 and 310 K. The coverage dependence requires that there be increasing numbers of islands per terrace at decreasing coverages. The temperature dependence of the overlayer was measured also. The frequency of the frustrated translational motion of the CO admolecules parallel to the surface is estimated to be 45 cm−1. At fractional (ϑ) coverages up to 1/6, the ordered islands of CO disorder substantially below the desorption temperature. At ϑ = 1/3, disordering is much less marked. The disordering behavior depends strongly on the distribution of sizes of islands in the overlayer. The correct distribution was determined and used to calculate the island sizes as a function of coverage. The mean number of CO molecules per island is 1000 at ϑ = 1/6, 500 at ϑ = 0.14, and 300 at ϑ = 0.12.