ionisations

Ionisation is the process by which atoms or molecules lose one or more electrons, forming positively charged ions. In atoms, electrons are bound by energy barriers called ionisation energies; the first ionisation energy removes the outermost electron, and further ionisations require progressively higher energies, creating ions with greater positive charge. Ionisation plays a central role in producing plasmas and enabling many analytical techniques.

Ionisation occurs through several mechanisms. Photoionisation happens when photons carry sufficient energy to overcome the binding energy of an electron. Collisional or impact ionisation occurs when energetic particles, such as electrons, collide with atoms or molecules and eject electrons. Field ionisation involves strong electric fields that distort or suppress the binding potential, allowing electrons to escape. In many environments, multiple mechanisms operate together, and the rate of ionisation depends on the energy distribution of the incoming particles and the surrounding conditions.

In gases and plasmas, ionisation states reflect the balance between ionisation and recombination processes. In thermal equilibrium, the Saha equation provides the relative populations of consecutive ionisation stages as a function of temperature and electron density; in non-equilibrium plasmas, time-dependent rate equations describe how ionisation states evolve.

Applications of ionisation include detectors such as ionisation chambers and mass spectrometers, where ions are produced and measured. In astrophysics, ionised gas produces characteristic spectral lines used to study stars and the interstellar medium. Ionisation also affects atmospheric electricity and is central to plasma processing in semiconductor manufacturing.