ionizations

Ionization is the process by which atoms or molecules acquire a net electric charge by loss or gain of electrons, creating ions. In most contexts, ionization refers to electron removal from neutral species, producing cations; electron attachment creates negative ions.

Primary mechanisms include photoionization, where photons with sufficient energy eject electrons; collisional ionization, where energetic particles collide to eject electrons, common in plasmas; field ionization, which uses strong electric fields; and tunnel ionization at high field intensities. Ionization also occurs in inner shells or valence electrons, with differing energy thresholds.

Ionization energy or ionization potential is the energy required to remove the outermost electron; successive ionizations require higher energies (second, third, etc.). The ionization state (charge) of an atom is denoted by Roman numerals in spectroscopy (e.g., Fe II). Ionization cross-sections describe the probabilities of ionization as a function of energy.

Ionization is critical in astrophysics, laboratory plasmas, atmospheric chemistry, and analytical techniques such as mass spectrometry. In many environments, ionization balance results from competing ionization and recombination processes, leading to characteristic ion populations that depend on temperature, density, and radiation fields.

The study of ionization underpins understanding of spectra, plasma behavior, and chemical reactivity, as well as the interpretation of signals in detectors and space observations.