Origin of the Coverage-Dependent Vibrational Shift for O on Ni(100)

The p(2×2) and c(2×2) coverages of O on Ni(100) are studied with a cluster model. The p(2×2) coverage is modeled by the addition of a single O at a fourfold site. The c(2×2) is modeled by the explicit inclusion of four additional O atoms. Consistent with electron energy-loss spectroscopy results, the O-Ni vibration is 85 cm^-1 lower for the c(2×2) cluster than for the p(2×2). The height of O above the surface is 0.11 {} smaller for the c(2×2) coverage. The nature of the O-to-Ni bonding is similar for both coverages.