Adsorbate resonant states: Resonance energy shifts due to elastic multiple electron scattering

We show that the energy of adsorbate resonant states observed in a number of electron spectroscopies is profoundly affected by the multiple scattering of the incident and/or emitted electrons participating in the resonance. High-resolution electron-energy loss spectroscopy is used to investigate the X ³Σg^-->a ¹Δg electronic excitation of O₂ physisorbed in two different monolayer phases on graphite. The observed 2-eV shift in the resonance energy between the {ζ} 2 and {δ} phases is shown to be produced by elastic multiple scattering within the molecular overlayers. This phenomenon, in other electron spectroscopies, will lead to disagreement in the location of resonance energies as determined by different experimental techniques.