Oxygen adsorption on Ag(110) is investigated as a function of coverage by high resolution electron energy loss spectroscopy. We find for the O-Ag(110) stretching frequency a value of 39.1 meV from the limit of low coverage up to the (4 x 1) structure. The frequency is then gradually shifted upwards, whereby an increase of 0.9 meV is observed when the reconstruction changes from the (3 x 1) to the (2 x 1) phase. We show that the shift is explained by a dipolar theory. The dependence of the frequency on coverage is discussed with reference to the model recently proposed by Schimizu and Tsukada [Surf. Sci. Lett. 295 (1993) L1017] who attributed the shift to fine changes in the atomic positions.
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