XPS as a Probe of Gap Opening in Many Electron Systems
  Core hole photoemission (XPS) provides a powerful indirect probe of the low energy excitations of a many electron system. We argue that XPS can be used to study the way in which a gap opens at a metal-superconductor or metal- insulator transition. We consider the "universal" physics of how the loss of low energy excitations modifies XPS spectra in the context of several simple models, considering in particular the case of a two dimensional d-wave superconductor. 
Robert Haslinger and Nic Shannon