1. A study of the energy of photo-electrons or Auger electrons (a two-electron process) emitted when a substance is bombarded with electrons, ions, or electromagnetic radiation.
2. The study of the energy spectra of photo-electrons or Auger electrons that are emitted from a substance upon bombardment by electromagnetic radiation, electrons, or ions.

It is used to investigate atomic, molecular, or solid-state structure, and in chemical analysis.