CO overlayers on Ru(0001) studied by helium atom scattering: Structure, dynamics, and the influence of coadsorbed H and O

Ordered phases of CO on a Ru(0001) surface have been characterized with regard to structural and dynamical properties using high resolution helium atom scattering. In the energy regime below 10 meV a vibrational mode corresponding to a frustrated translation parallel to the surface (⁠$FTx)$ could be identified, the energy amounts to 5.9 meV for isolated CO molecules and to 5.75 meV for the $(∛×∛)R30°$CO structure. The formation of the more compressed $(2∛×2∛)R30°$CO and $(5∛×5∛)R30°$CO structures is accompanied by significant changes of the low energy external vibrations, in pronounced contrast to the gradual frequency increase of the CO internal $ν1$-vibration. Coadsorption of hydrogen or oxygen was found to result in substantially larger $FTx$ energies. Implications of these findings on the character of the molecule-surface interaction will be discussed, as well as the connection between the $FTx$-dispersion and the strength and type of the adsorbate-adsorbate interaction.