Experimental observation of vibrational modes on Ag(111) along ΓM and ΓK

We present an off-specular high-resolution electron energy-loss spectroscopy study of the vibrational modes along the ΓM and ΓK directions in the surface Brillouin zone of clean Ag(111). We show experimental data of vibrational modes at the K̄ edge, thereby extending the earlier He-scattering data on the S₁ Rayleigh mode and pseudo-Rayleigh mode. In addition, we observed the S₃ gap mode and three high-frequency modes along ΓK. According to our knowledge, this is the first experimental evidence for the existence of these high-frequency modes in the (111) surface Brillouin zone of fcc metals. Along ΓM the experimental dataset of the S₂ gap mode is extended to smaller Q_//, and for the first time the high-frequency vibrational mode close to the top of the projected bulk phonon band is observed. The EELS data is analyzed by comparison to theoretical studies previously reported in the literature. This paper presents new experimental data of the surface vibrational structure, which can stimulate theoreticians to refine their modeling and improve the understanding of the Ag(111) surface lattice dynamics.