A He atom scattering study of the low energy vibrations of CO adsorbed on Ni(110)

The low energy vibrations of CO molecules adsorbed on Ni(110) have been studied with high resolution He atom scattering. For coverages below about theta=0.3 a frustrated translation (T) mode with an energy of 3.9 meV has been measured which is assigned to an on-top site adsorption. For the (2 x 1) saturation structure where the CO molecules occupy tilted bridge sites the T-mode energy amounts instead to 7.5 meV. Although the CO tilting is presumably caused by a CO-CO repulsion, the T-mode energy exhibits no dispersion with the parallel momentum transfer which would suggest that the (dipole-dipole) repulsion has only a negligible effect on the T-mode vibrations. (C) 1997 Elsevier Science B.V.