ELECTRONIC EXCITATION OF O-2 MOLECULES PHYSISORBED ON AG(110)

We report a high-resolution electron-energy-loss study of the X(3) Sig ma(g)(-)-->a(1) Delta(g) electronic excitation of O-2 molecules physis orbed on Ag(110). Three resonances are observed to occur at incident e lectron energies of approximate to 6, 8.5, and 12 eV in the monolayer regime. These features shift to higher energies by about 1-1.5 eV with increasing coverage. The relative intensities of these peaks also cha nge dramatically as a function of coverage. This allows us to explore the appearance of resonant states which are not seen in the gas-phase molecule, and which can be compared with similar features observed in electron-stimulated desorption studies of condensed O-2.