On the Ru(0001) surface Cs and CO form a very well ordered (Cs + CO)-(2 × 2) compound layer whose structure was determined recently (Cs on-top, CO in 3-fold sites). Here we present a vibrational analysis of the same system using high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and low-energy electron diffraction (LEED). Two C-O stretch frequencies are observed depending on whether there are one or two CO molecules in the 2 × 2 cell. Two CO-Ru stretch modes are observed for the first time and are assigned to adsorption in the hcp and fee hollow sites within the 2 × 2 cell. They are very weak in intensity which is attributed to the threefold-hollow site and some screening in the 2D compound. With CO adsorption a change of the electronic structure is observed: the adlayer loses metallicity and the Cs-Ru stretch becomes visible. Strong changes of the Cs-Ru stretch energies are observed with CO coverage.
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