Phase transition of dissociatively adsorbed oxygen on Ag(001)

We show that dissociative oxygen adsorption on Ag(001) induces below room t emperature a missing row 2 root 2x root 2 reconstruction of the substrate. As demonstrated by the analysis of the photoelectron diffraction patterns, the oxygen atoms sit thereby in a c(2x2) arrangement in the previous fourfo ld hollow sites nearly coplanar with the Ag atoms, while rows of substrate atoms are removed along the [100] directions. Annealing the crystal above 3 50 K restores the p(1X1) symmetry and the oxygen moves to 0.6 Angstrom abov e the fourfold hollow site. It becomes then more oxidic in nature, as demon strated by the shift of the O1s level from 530.3 eV to 528.3 eV. The phase transition affects also the O2s and O2p levels as well as the surface compo nent of Ag 3d(5/2). The vibrational frequency of the oxygen adatoms against the surface decreases at the phase transition, in accord with the larger a dsorption distance. The highs temperature phase is active towards CO and C2 H4 oxidation, while the low-temperature phase is nor. When cooling the samp le below room temperature the reconstructed phase is restored. The time con stant of this process as well as the chemical reactivity of the high-temper ature phase are weakly reproducible since they depend on the previous histo ry, i.e., presumably on the subsurface oxygen content of the sample.