Vibrational analysis of cesium on Ru(0001)

We report a high-resolution electron-energy-loss spectroscopy study of the coverage dependence of the Cs-Ru stretch vibration at 300 K. Surprisingly, the frequency of Cs-Ru stretch vibration shifts upward by about 30% with Cs coverage for $0<~{\theta }_{\mathrm{Cs}}<~0.19$. Only a quarter of this shift can be explained through lateral dipole-dipole interaction. We discuss two further mechanisms that may cause the other part of the shift. A slight decrease of the stretch frequency for ${\theta }_{\mathrm{Cs}}>0.19\mathrm{}$ is associated with the metallization of the alkali-metal layer including a structural change.