RESONANCE ELECTRON-SCATTERING BY PHYSISORBED AND CHEMISORBED O-2 ON PT(111)

We have investigated resonance electron scattering by both physisorbed and chemisorbed molecular O2 on Pt(111). In each case, the molecular sigma-shape resonance is manifest in the vibrational excitation cross section but the resonance energy is lowered in comparison with the fr ee molecule. We attribute the shift in energy (2.25 eV) in the physiso rbed molecule primarily to an image potential effect. We identify the increased bond length of the chemisorbed molecule as an important fact or in the larger shift (5.75 eV) observed in this case. This conclusio n is supported by a consideration of data for adsorbed molecules conta ining the CO group.