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A new physical mechanism is proposed to explain the shifts in vibrational frequency of negatively adsorbed species on a metal surface. Ab initio cluster model calculations for NO adsorbed Dn Ag(111) in two different orientations, N-down and O-down, suggest that the low-coverage HREELS peak appearing at 1282 cm-1 is due to adsorbed NO in an O-down orientation. The large shift in vibrational frequency relative to free NO is shown to be due to the internal electric field produced by the negative charge on the adsorbate and the metal image charge. While external electric fields are known to produce vibrational shifts this is the first evidence that the same effect can be produced by internal electric fields which have their origin in the ionic nature of the chemisorption bond between NO and Ag(111).

We report results from ab initio cluster-model calculations on the O(1s) binding energy (BE) in the alkaline-earth oxides, MgO, CaO, SrO, and BaO; all these oxides have a cubic lattice structure. We have obtained values for both the initial- and final-state BE's. A simple point-charge model, where an O2-anion is surrounded by point charges, accounts for the observed shift in the O(1s) BE by about 2.5 eV to lower energy going from MgO (largest BE) to BaO (smallest BE). This point-charge model only describes the effect of the Madelung potential at the 1s ionized O2- anion; it does not allow any covalent bonding between the metal (M) cations and the oxygen anions. Once the effect of the O2 --> M2+ covalent bonding is included explicitly by using O(m)M(n) cluster models of the MO crystal, the trend in the O(1s) BE to smaller values for heavier metals does not change significantly. This shows that the main contribution to the shift is not the different amount of covalent mixing in the alkaline-earth oxides but rather the change in the Madelung potential along the group.