Oxygen vibrations in O-Ag(001)

The vibrational modes of oxygen on Ag(001) are studied both theoretically, by density functional perturbation theory (DFPT) within the local density approximation, and experimentally, by means of high resolution electron energy loss spectroscopy (HREELS). The frequencies of the O-Ag stretch mode and of the parallel modes are calculated for the p(1 x 1), c(2 x 2), and p(2 x 2) geometries of oxygen on the non-reconstructed Ag(001) surface. Increasing the coverage decreases the chemisorption energy as well as the O-Ag stretch frequency. The vibrational frequencies of O in the 2root2- x root2 missing row reconstructed structure, the phase indicated by X-ray photoelectron diffraction as the low temperature phase of this system, are also investigated. The comparison of the DFPT calculations with the HREELS data gives further support to the existence of a missing row structure.