At this point, it is interesting to analyze the possible mechanism yielding the formation of such structures by the incorporation of atomic oxygen into the oligomers. The first step has to be the formation of atomic oxygen by the dissociation of molecular oxygen on the surface. It has been reported that O2 adsorption on Au(111) presents a very low yield , being necessary to add atomic O to improve such adsorption, while O2 readily dissociates on a silver surface . To confirm this point, we have carried out a theoretical estimation of the dissociation barriers for O2 on both substrates, yielding values of 1.26 eV for Ag(111) and 1.90 eV and Au(111), in excellent agreement with previous theoretical findings . Consequently, the dissociation of molecular O2 on Au(111) is expected to be nearly negligible whereas it remains feasible on Ag(111). The second step would correspond to the replacement of the metallic center by the atomic oxygen. Our DFT results indicate that the stability of the Au metal-organic oligomers is higher than those stabilized by Ag atoms (-2.19 eV vs -1.64 eV, respectively), making more difficult for the oxygen atom to displace the metallic center. Thus, the following scenario can be envisioned: when molecular oxygen reaches the Ag(111) surface at RT, it dissociates into atomic oxygen that it is available to displace the metal adatom in the oligomers, while this process is quenched on Au(111). To analyze the stability of such phase and to confirm our hypothesis, we have post-annealed the exposed surface to 370 K. The C 1s spectrum is preserved, with the only difference of the disappearance of the grey component at 284.5 eV that is assigned to carbon contaminants physisorbed on the surface. The O 1s peak also keeps its two components although the one at 530.5 eV is significantly reduced. This can be easily understood if we notice that this component has a double contribution, AgO and physisorbed O2, and that the second one can desorb upon annealing, thus remaining the one associated to AgO, which is known to desorb at temperatures above 500 K .