Bremsstrahlung (braking radiation) is electromagnetic radiation produced when a charged particle decelerates, when its course is changed by another charged particle. A photon is emitted that balances the energy equation. The resulting radiation emission (from a gas/plasma) has a continuous spectrum (continuum emission), as opposed, for example, to emission lines. It is also called free-free radiation, expressing the point that its production does not involve an electron bound, before or afterward, to an atom. The term electron-ion bremsstrahlung (e-ion bremsstrahlung) refers to bremsstrahlung caused by a free electron encounter with an ion and electron-electron bremsstrahlung (e-e bremsstrahlung) is that between two electrons. Other types of generation include bremsstrahlung induced during beta decay, and bremsstrahlung due to interaction between an atom's orbiting (bound) electron and a passing charged particle.
An astronomical source of bremsstrahlung is the intracluster medium associated with a galaxy clusters.
The reverse of bremsstrahlung radiation is also possible, called free-free absorption or inverse bremsstrahlung, where an electron and another particle interact and a photon is absorbed. Free-free absorption occurs in HII regions, affecting their spectrum.