Rydberg states with core hole

XAS, on the other hand has a core-excited final state for which the effect of the core-hole must be taken into account. To obtain the full spectrum, i.e., valence, Rydberg and continuum excitations, we use the Slater transition-state approach [22,23] with a half-occupied core-hole. This provides a balanced description of both initial and final states allowing the same orbitals to be used to describe both initial and final states and all transitions are obtained in one calculation [23,24]. Details of the computational procedure can be found in the original papers as referenced in the following sections. In the present chapter, the focus is on the surface chemical bond and the spectra, measured or calculated, will mainly be used to obtain the required information on the electronic structure. 

Below the ionization edge, features in the absorption spectra called discrete resonances are due predominantly to transitions of the core electron to bound states that result from configurations in which the core electron has been promoted to a valence or Rydberg orbital or an appropriate linear combination. These states are degenerate with the ionization continua of other configurations and therefore can autoionize in an Auger-like process where one electron falls into the core hole and another electron is emitted. 

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