SURFACE DYNAMICS OF OXYGEN-COVERED NI(100)

Ordered overlayers of oxygen atoms on Ni(100) have been investigated by inelastic scattering of He atoms. For the O-p(2 x 2) overlayer and the O-c(2 x 2) overlayer the surface-phonon dispersion curves of the Rayleigh mode have been determined for both high symmetry directions. Along the [100] azimuth of the c(2 x 2) overlayer a large gap of 3 meV between the two branches of the Rayleigh mode at the zone-boundary has been observed; this gives evidence for a very strong repulsive O-O interaction in the rather densely packed O-c(2 x 2) phase. Additionally, a flat phonon mode at about 10 meV has been observed for both high-symmetry directions of the O-c(2 x 2) phase, which was not resolved in earlier EELS measurements. Lattice-dynamical calculations were performed to interpret this behaviour and suggest that the flat mode corresponds to localized vibrations at defects within the O-c(2 x 2) overlayer.