When a primary beam electron strikes an atom's orbital electron with enough energy to overcome the energy holding the orbital electron in the atom (absorption edge energy) the orbital electron can be ejected from the atom.
Then, for a brief moment in time (about 10^-12 sec) a vacancy exists in the atom.
The inner orbital vacancy is quickly filled by an outer orbital electron, which loses some of its kinetic energy as it falls to the inner orbital.
The emitted energy, a photon, is shown in red. If the emitted photo has an energy between 0.124 and 124KeV, it is termed an X-ray. The energy, and wavelength, of the emitted photon is characteristic of the element.
X-rays are used to determine the chemical composition of the sample. X-rays are a part of the electromagnetic spectrum and as such have the properties of waves, and of particles of energy. These two characteristics of the electromagnetic spectrum are inversely related via this simple expression:
E = 12.4 / λ
where
E = energy (keV)
λ = wavelength (A)
Beam electron
Scattered beam electron
Ejected orbital electron
Emitted Photon