Anharmonicity of the O-2-Ag(001) chemisorption potential

The coverage dependence of the vibrational modes of O-2 chemisorbed on Ag(001) was studied by electron energy loss spectroscopy. We observe the dipole active modes associated to the intramolecular stretch and to the molecule-surface vibrations. Moreover two further features are present in the EEL spectra, due to the anharmonicity of the molecular potential: the combination-band of the stretching and of the molecule-surface vibration, and the first overtone of the O-O stretching mode. From the EELS intensity of the O-O stretching mode versus coverage the dynamical charge transfer e* = 0.61 +/- 0.02 e(-) is determined which corresponds to a dynamical dipole moment mu=0.16+/-0.01 D. The O-2-Ag vibrational frequency is found to decrease with coverage, suggesting that a weakening of the molecule-surface bond takes place. This observation is confirmed by thermal desorption spectroscopy. The anharmonicity parameter x(e) of the O-O potential increases significantly upon chemisorption, in analogy to O-2-Ag(110) and O-2-Pt(111). The intensity of the overtone peak is analyzed within the Morse approximation, allowing us to discriminate the contributions arising from the mechanical anharmonicity, and from the nonlinearity of the dynamical charge flow. A correlation is observed between the magnitude of e* and the contribution of the latter: term to the overtone intensity.