In an XPS experiment, a sample is irradiated with monochromatic X-rays, typically Al Kα or Mg Kα. The energy of these X-rays is sufficient to eject core-level electrons from the atoms in the sample. The kinetic energy of these emitted electrons is measured using an electron spectrometer. By knowing the incident X-ray energy and the measured kinetic energy, the binding energy of the electrons can be calculated. This binding energy is characteristic of specific elements and their chemical states, allowing for detailed surface analysis.
