Adsorption of CO on Cs-Precovered Ru(001): Formation of a Double-Layer Structure Cs/Cs + CO/Ru and Complete Screening of the CO Dipole

H. Kondoh and H. Nozoye
[Surf. Sci., Vol.334, pp.39-54, 1995]

Adsorption of CO on Cs-precovered Ru(001) with one-monolayer coverage (thetaCs = 0.33) has been investigated by means of electron energy loss spectroscopy (EELS), temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). We found unusual adsorption of CO on the Cs(1 ML)/Ru(001) surface. Gaseous CO adsorbs on the completely Cs-precovered surface at 85 K to a saturation coverage of 0.23. Although the low-resolution EELS and UPS results indicate that CO molecularly adsorbs on this surface, the C-O stretching mode is not observed in the specular mode of high-resolution EELS but clearly appears at quite a low frequency of 1245 cm^-1 in the off-specular mode as shown in Fig.1. From a combination of the EELS, TPD and XPS experiments, we propose an adsorption model in which the CO molecules displace one-fourth of the Cs adatoms from the preadsorbed Cs monolayer, resulting in formation of a double-layer structure, Cs/Cs + CO/Ru(001); the displaced Cs atoms adsorb on the "rotated-p(2 x 2)" lattice which is composed of Cs and CO with a ratio of 1:1. In this adsorption state, it is considered that the CO species has an inclined or a side-on-bonded geometry and that the CO dipole is effectively screened by the second-layer Cs. This complete screening of the CO dipole is explained in terms of combines effects of the embedding of CO into the double-layer structure, a large polarizability of Cs and laterally delocalized electrons in the double-layer structure.

Fig. 1. High-resolution EEL spectra for CO_sat/Cs(1 ML)/Ru(001) in the in-specular mode (theta_i = 60°, theta_s = 60°) and the off-specular mode (theta_i = 51° , thetas = 60°).