2. The fluorescent atom

When an electron from the deeper atomic layers is ejected from its orbital during a collision, the resulting excitation of the electron pattern can lead to characteristic X-ray emission. X-ray fluorescence emission can be thought of as a two-stage process: ionization, followed by X-ray emission. With very few exceptions (see note 1), the ionized atom has no "memory" of how it came to be in this state, whether through interaction with a proton, electron or X-ray photon. We will therefore briefly review the principles of X-ray emission, which is totally independent of the initial ionizing phenomenon.