Ionised

Ionised is the British spelling of ionized, describing atoms or molecules that have lost or gained electrons, resulting in a net electrical charge. Ionisation typically produces cations (positive charge) when electrons are removed, or anions (negative charge) when electrons are added. The term is used in physics, chemistry, and related fields, and is often contrasted with the neutral state.

Ionisation occurs when sufficient energy is transferred to an atom or molecule to overcome its ionisation energy. Ionisation can occur via various mechanisms: photoionization by photons, such as ultraviolet radiation; electron impact or collisional ionization in hot gases; ionization by strong electric or magnetic fields (field ionization); and chemical or plasma processes such as electron transfer in plasma reactors and mass spectrometry ion sources.

In many contexts, ionised gas refers to plasma, a state of matter in which electrons are not bound to atoms. Plasmas are highly conductive, respond to electromagnetic fields, and contain ions and free electrons in varying proportions. The degree of ionisation is a measure of the fraction of atoms that are ionised.

Natural and laboratory contexts: in astrophysics, H II regions around hot, young stars consist of ionised hydrogen; the solar corona and other stellar atmospheres contain highly ionised plasma. In chemistry and industry, ionisation is used in mass spectrometry, surface processing, semiconductor manufacturing, and lighting (plasma displays, fluorescent lamps). In flames, characteristic emission lines arise from excited ions and atoms as they ionize and de-excite.

The term ionised is closely related to ionization and ionizing processes, and confusion can arise with the orthographic variants ionised (British) and ionized (American).