The angular dependence of plasmon loss features in XPS spectra from polycrystalline aluminum: Clean surfaces and effects of oxygen adsorption

The angular dependence of relative intensities of bulk- and surface-plasmon loss peaks associated with X-ray photoemission from the Al 2p core level has been measured for aluminum samples under ultrahigh vacuum conditions. At near-grazing electron exit angles, a sixfold increase in the single-surface-plasmon relative intensity is observed, while the single-bulk-plasmon relative intensity is decreased by ～20%. These experimental results are compared with theoretical calculations based on a dispersionless-plasmon jellium model and including both extrinsic and intrinsic plasmon creation. The presence of intrinsic creation is suggested and theoretical relative intensities are in good agreement with, although generally somewhat higher than, corresponding experimental values. For specimens with low coverages of oxygen (～0.15 monolayer), only surface-plasmon loss features are found to be associated with the O 1s core level, indicating near-surface adsorption.