Jonisation

Jonisation, sometimes rendered as ionisation in English, is the process by which atoms or molecules acquire a net electric charge by losing or gaining electrons. In common usage, ionisation refers to electron removal from a neutral species, producing a positively charged ion, while electron attachment yields a negative ion. The process is fundamental to many physical, chemical, and astronomical phenomena.

Several mechanisms drive ionisation. Photoionisation occurs when photons have enough energy to overcome the ionisation threshold. Electron impact ionisation, or collisional ionisation, happens in hot gases or plasmas when energetic electrons eject electrons from atoms. Field ionisation involves strong electric fields that reduce the Coulomb barrier, enabling tunneling or over-the-barrier emission. Autoionisation is the spontaneous ejection of an electron from an excited state before radiative relaxation.

Ionisation energy, or ionisation potential, is the energy required to remove the first electron; successive ionisation energies increase as electrons are removed. In many environments, the balance between ionised and neutral species depends on temperature and density; the Saha equation gives the ratio for systems in thermodynamic equilibrium.

In plasmas and astrophysical contexts, ionisation shapes radiation transport, chemistry, and dynamics of stars, nebulae, and the interstellar medium. In chemistry, ionisation is central to reaction mechanisms and mass spectrometry sample preparation; various ionisation methods are used to generate ions for analysis, such as electron impact, electrospray, and matrix-assisted laser desorption/ionisation.

Ionisation detection and measurement rely on counting charges or measuring ionisation cross sections, ion currents, or emitted radiation. Understanding ionisation processes helps interpret spectra, model laser–matter interactions, and design devices that rely on charge carriers or controlled plasma formation.