Electron-Energy-Loss Spectroscopy of Ordered Oxygen Overlayers on Ru(001)

High-resolution electron-energy-loss measurements and relevant theoretical calculations for the vibrational structure of ordered oxygen overlayers on the Ru(001) surface are presented. The vibrational spectrum of the $p(2\mathrm{}\times 2)$ overlayer shows a single dipolaractive mode above the substrate phonon band. Because of a reduction in symmetry of the threefold adsorption site, however, the vibrational spectrum of the $p(1\mathrm{}\times 2)$ overlayer is fundamentally different, showing two dipolar-active modes above the substrate phonon band.