Bremsstrahlung is electromagnetic radiation emitted when a charged particle accelerates during interaction with another charged particle or field. The microscopic process creates a photon, while its inverse absorbs one. Thus it contributes to a photon-number collision source in the photon number current, unlike ordinary Compton scattering. Thermal bremsstrahlung averages this emission over a thermal distribution of charged particles.
Radiation from unbound charged particles during a scattering encounter, notably electrons accelerated by ions in a plasma. Both the initial and final electron states are free, distinguishing this process from emission involving an atomic bound state.
For an electron accelerated through a electric potential difference and stopped at a massive target, the largest single photon energy is approximately . Conservation of energy and give the short-wavelength cutoff . Target recoil is neglected in this limiting relation.
Free-free absorption is the inverse of bremsstrahlung: a free electron absorbs a photon during an encounter with an ion, which permits conservation of momentum. In local thermodynamic equilibrium its absorption coefficient and the thermal free-free emissivity obey Kirchhoff's law of thermal radiation, . For a nonrelativistic plasma at , , including the stimulated-emission correction in the net coefficient.
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Bremsstrahlung is a German term that translates to "braking radiation." It refers to the electromagnetic radiation emitted when charged particles, such as electrons, are accelerated or decelerated, particularly when they pass near atomic nuclei. This process occurs because the change in the velocity of the charged particle results in the emission of energy in the form of radiation, typically X-rays.