Using electron energy loss spectroscopy and thermal desorption spectroscopy, we show that the adsorption of ${\mathrm{O}}_2$ on Ag(110) creates two distinct molecular chemisorbed species. Only one, $\alpha -{\mathrm{O}}_2$, can be accessed by warming (converting) the physisorbed state and, unlike $\beta -{\mathrm{O}}_2$, does not dissociate to atomic O at higher temperatures. With the aid of molecular dynamics simulations, we propose that the lateral interaction between neighboring ${\mathrm{O}}_2$ molecules in a dense physisorbed layer stabilizes the appropriate precursor orientation to enable the conversion into $\alpha -{\mathrm{O}}_2$ to occur.

• Received 10 September 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.5224