Oxygen adsorption on Ag(111)

We have investigated the interaction of oxygen with Ag(111) by using a supersonic molecular beam in the impact energy range 93-800 meV. At 105 K, contrary to the results of Carley et al. [Surf. Sci. 238 (1990) L467], we find no evidence for O-2 adsorption even after very high O-2 exposures (similar to 25000 L) indicating that for a clean surface the sticking probability S is lower than 6 x 10(-7) far the whole impact energy range. At room temperature dissociative oxygen adsorption occurs at E(i) = 0.80 eV, with S approximate to 9 x 10(-7). The data show however evidence that the adsorption process is mediated also in this case by adsorbed OH so that S is even smaller for the clean surface.