Helium atom scattering study of the frustrated translation mode of CO adsorbed on the Cu(001) surface

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Abstract
The structure and vibrational modes of CO adsorbed on Cu(001) have been investigated by helium atom scattering (HAS). Below coverages of θ=0.13, CO adsorbs as isolated molecules, and the time of flight (TOF) spectra of helium atoms scattered from the surface are characterized by energy losses and gains of 3.94 ± 0.07 meV. With the aid of isotope shift measurements, this mode is unambiguously assigned to the frustrated translation mode of the adsorbed CO. In the c(2×2) structure (θ=0.5) this mode is found to show dispersion in both the 〈100〉 and 〈110〉 azimuths with frequencies ranging from 3.9 meV at the zone origin to 5.6 meV at the zone boundary. A simple force constant model with a single nearest neighbor force constant within the layer is found to be sufficient to describe the dispersion. For coverages slightly less than θ=0.5 the angular distributions of scattered helium atoms show features characteristic of vacancies in the overlayer. In the TOF spectra an additional nondispersive mode at about 4 meV is also seen and assigned to the frustrated translation of CO molecules adjacent to the vacancies in the c(2×2) overlayer. The Rayleigh mode is clearly observed in the c(2×2) structure but is found to decrease slightly in frequency while the longitudinal resonance, which is especially prominent on the clean surface, disappears completely. The results are interpreted with the aid of slab calculations.