HREEL SPECTRA OF VARIOUS OXYGEN STRUCTURES ON RH(110)

High-resolution electron energy loss spectroscopy and low electron energy diffraction have been used to study ordered and disordered layers of oxygen adsorbed on the Rh(110) surface at different temperatures. It has been found that at temperatures of 90-120 K, low coverages of disordered atomic oxygen produce a single vibrational mode at 72 meV which is replaced by two losses at 45 and 63 meV when an ordered (2 x 1)p2mg monolayer is formed. Losses at 47 and 65 meV characterize the adsorbed oxygen in the (2 x 2)p2mg, (2 x 3)p2mg, and c(2 X 2n) structures produced at elevated temperatures, which involve reconstruction of the substrate. The changes in the vibrational spectra are explained in the framework of structural models where oxygen occupies different adsorption sites,