Abstract
The dispersion of adsorbate and substrate associated modes of the Cu(100) surface covered with a p(2×2) sulfur overlayer has been studied along the ¯ΓX and the ¯ΓM direction by inelastic electron scattering. The Rayleigh mode (S4 in the ¯ΓX and S1 in the ¯ΓM direction) is only slightly changed by the sulfur overlayer. One adsorbate-induced phonon branch above the bulk phonon band is fond. This mode shows a dispersion of 35 cm−1 in both directions and is attributed to the perpendicular vibration of the adatoms. Lattice dynamical calculations show that the parallel vibration mode has no dispersion throughout the two-dimensional Brillouin zone. For a perpendicular distance of 1.39 Å of the sulfur atoms to the first Cu layer the frequency of the parallel and perpendicular mode is almost the same (300 cm−1) in the middle of the Brillouin zone.
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