SYMMETRY AND STRUCTURE OF THE RECONSTRUCTED NI(110)-(2X1)O SURFACE

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 O16 by O18 discriminates adsorbate from substrate phonons. With the scattering plane along [110] 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 Cs with the mirror plane oriented along [110]. 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 [110] or the [110] direction. © 1990.