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

The $p(2\mathrm{}\times 2)$ and $c(2\mathrm{}\times 2)$ coverages of S on Ni(100) are studied with a cluster model. Consistent with a recent lattice-dynamics study, the chemical contribution (rigid Ni lattice) to the coverage-dependent shift of the S vibrational energy is of opposite sign to that for O. Analysis of the cluster wave functions suggests that S becomes less negatively charged with coverage, while O becomes more ionic; however, this effect is small.