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. © 1984 The American Physical Society.