High-resolution electron energy-loss spectroscopy (HREELS) has been used to investigate the vibrational properties of the newly discovered, well-ordered oxygen (1 × 1) overlayer on Ru(001). The totally symmetric Ru-O stretch has been mapped out along the Γ̄-K̄-M̄-K̄direction of the surface Brillouin zone (SBZ). The stretching mode exhibits a strong downward dispersion of about 100 cm-1along Γ̄-K̄ starting at 660 cm-1, with a subsequent upward dispersion along K̄-M̄. The dispersion observed for small k∥is explained by dominating long-range dynamical dipole-dipole coupling. From comparisons of experimental data in the vicinity of the Γ̄ point with calculations for dipole-dipole coupling, the frequency of the isolated oscillator (singleton frequency), as well as the vibrational and electronic polarizabilities, can be estimated to be 598 cm-1, 0.8 angstrom3and 1.7 angstrom3, respectively. However, the short-range interactions deviate significantly from dynamical dipole-dipole interactions as is documented by the discrepancy between the calculated and the observed dispersion between K̄ and M̄.
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