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 (root 3x root 3)R30 degrees CO structure. The formation of the more compressed (2 root 3x2 root 3)R30 degrees CO and (5 root 3x5 root 3)R30 degrees 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 nu(1)-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. (C) 1997 American Institute of Physics.