text: In some cases, energy loss features due to plasmon excitations are also observed. This can either be a final state effect caused by core hole decay, which generates quantized electron wave excitations in the solid (intrinsic plasmons), or it can be due to excitations induced by photoelectrons travelling from the emitter to the surface (extrinsic plasmons).
  Due to the reduced coordination number of first-layer atoms, the plasma frequency of bulk and surface atoms are related by the following equation:  
  so that surface and bulk plasmons can be easily distinguished from each other.
  Plasmon states in a solid are typically localized at the surface, and can strongly affect IMFP.