ESCA is based on the observation of the electrons emitted under irradiation by photons.
The process is schematically described in Figure 2-1. The sample, placed in an ultrahigh vacuum, is irradiated with photons from a soft X-ray source with a well defined energy. Aluminium K_ X-rays (1486.6 eV) or magnesium K_ X-rays (1253.6 eV) are commonly used sources of radiation. These photons penetrate from 1 to 10 μm into a solid and interact with atomic and molecular orbital electrons in the surface