Abstract
We have measured the dispersion of oxygen and nickel vibrational modes of the reconstructed Ni(110)-(2 × 1)O surface along the high symmetry directions using momentum-resolved electron energy loss spectroscopy (EELS). Observation of the frequency shifts upon substitution of O 16 by O 18 discriminates adsorbate from substrate phonons. With the scattering plane along [ 1 10] two oxygen modes are observed, which are also dipole active. Their frequency at the \\ ̄ gG point (at 120 K) is 239 and 382 cm −1 respectively. With the scattering plane aligned along [001] an additional third oxygen mode at about 800 cm −1 is observed off-specular only. According to EELS selection rules the symmetry of the surface is thus C s with the mirror plane oriented along [ 1 10]. This is in agreement with the “saw-tooth” reconstruction model proposed for this surface, however also with a “modified missing-row” model with the oxygen atoms moved out of the long-bridge site along either the [ 1 10] or the [1 1 0] direction.
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