Oxidation of polycrystalline tin by hyperthermal atomic oxygen: an investigation using electron energy-loss spectroscopy

X-ray photoelectron spectroscopy (XPS) spectral behavior of an initial discrete core-hole state embedded or nearly embedded in a continuum is studied by a Greenʹs function model calculation. The particle¯hole interaction U in the final two-hole one-particle state is included in the initial core-hole self-energy by the ladder and ring approximation. With an increase of U, the transition to an unoccupied (particle) state near the double-ionization threshold increases in the final state. With a further increase of U, the transition to an empty bound level below the Fermi level dominates. It is shown that even when the coupling between the initial discrete core-hole state and the final continuum state is strong there can be a sharp resonant state. When the coupling is weak, there is a possibility of the breakdown of the quasi-particle approximation for the initial core-hole state. U can substantially affect the initial core-hole spectral function.