Electronic properties of Cs+CO coadsorbed on the Ru(0001) surface

The variation of the Cs $6s$ and the Cs $5p$ emission in He^* and Ne^* metastable deexcitation spectroscopy (MDS) as a function of the CO exposure indicates a demetallization of the Ru(0001)–(2×2)-Cs and the $Ru(0001)–(√3×√3)R30°-Cs$ surfaces upon CO coadsorption. This observation corroborates a (substrate-mediated) charge transfer from the Cs atom to the 2π^* orbital of CO. With the Ru(0001)–(2×2)-Cs system even at CO saturation, MD spectra show emission associated with the Cs $6s$ state, indicating that the Cs atoms are not completely ionized. Exposing the $(√3×√3)R30°-Cs$-pre-covered Ru(0001) to CO, surplus Cs of the first layer is displaced into a second layer. In this way, CO molecules are able to be accommodated into the first layer. Desorbing this second layer Cs by heating the sample to 600 K produces a (2×2) structure with one Cs and CO in the unit cell as evidenced by MDS and low energy electron diffraction.